diff --git a/step/worldmodel.cc b/step/worldmodel.cc
index 8130c7f..07766a3 100644
--- a/step/worldmodel.cc
+++ b/step/worldmodel.cc
@@ -1,1093 +1,1106 @@
 /* This file is part of Step.
    Copyright (C) 2007 Vladimir Kuznetsov <ks.vladimir@gmail.com>
 
    Step is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.
 
    Step is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
 
    You should have received a copy of the GNU General Public License
    along with Step; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
 
 #include "worldmodel.h"
 
 #include "clipboard.h"
 #include "simulationthread.h"
 #include "worldgraphics.h"
 
 #include "settings.h"
 
 #include "worldfactory.h"
 #include <stepcore/world.h>
 #include <stepcore/xmlfile.h>
 #include <stepcore/eulersolver.h>
 #include <stepcore/collisionsolver.h>
 #include <stepcore/constraintsolver.h>
 #include <stepcore/types.h>
 #include <QApplication>
 #include <QItemSelectionModel>
 #include <QTimer>
 #include <QUndoStack>
 #include <QMenu>
 #include <KLocalizedString>
 
 class CommandEditProperty: public QUndoCommand
 {
 public:
     CommandEditProperty(WorldModel* worldModel, StepCore::Object* object,
                 const StepCore::MetaProperty* property, const QVariant& newValue, bool merge);
 
     int id() const Q_DECL_OVERRIDE { return _merge ? 1 : -1; }
     bool mergeWith(const QUndoCommand* command) Q_DECL_OVERRIDE;
 
     void redo() Q_DECL_OVERRIDE;
     void undo() Q_DECL_OVERRIDE;
 
 protected:
     /* It's important to properly compress commands
      *  or the stack becomes too big and slow */
     struct EditProperty {
         StepCore::Object* object;
         const StepCore::MetaProperty* property;
         QVariant oldValue;
         QVariant newValue;
     };
 
     bool _merge;
     WorldModel* _worldModel;
     QList<EditProperty> _commands;
     //QList<StepCore::Object*> _objects;
 };
 
 CommandEditProperty::CommandEditProperty(WorldModel* worldModel, StepCore::Object* object,
             const StepCore::MetaProperty* property, const QVariant& newValue, bool merge)
         : _merge(merge), _worldModel(worldModel)
 {
     //qDebug() << "CommandEditProperty: " << object->name() << " " << property->name() << " " << newValue.toString() << endl;
     EditProperty p = { object, property, property->readVariant(object), newValue };
     _commands << p;// _objects << object;
 }
 
 void CommandEditProperty::redo()
 {
     bool fullUpdate = false;
     foreach(const EditProperty& p, _commands) {
         if(p.newValue.type() != QVariant::String) p.property->writeVariant(p.object, p.newValue);
         else p.property->writeString(p.object, p.newValue.value<QString>());
         if(!p.property->isDynamic() || p.property->hasSideEffects()) fullUpdate = true;
     }
     //_worldModel->objectChanged(NULL);
     _worldModel->emitChanged(fullUpdate, true);
     //foreach(StepCore::Object* object, _objects) _worldModel->objectChanged(object);
 }
 
 void CommandEditProperty::undo()
 {
     bool fullUpdate = false;
     foreach(const EditProperty& p, _commands) {
         p.property->writeVariant(p.object, p.oldValue);
         if(!p.property->isDynamic() || p.property->hasSideEffects()) fullUpdate = true;
     }
     //_worldModel->objectChanged(NULL);
     _worldModel->emitChanged(fullUpdate, true);
     //foreach(StepCore::Object* object, _objects) _worldModel->objectChanged(object);
 }
 
 bool CommandEditProperty::mergeWith(const QUndoCommand* command)
 {
     const CommandEditProperty* cmd = dynamic_cast<const CommandEditProperty*>(command);
     Q_ASSERT(cmd != NULL);
     if(cmd->_commands.count() != 1) return false;
 
     const EditProperty& p1 = cmd->_commands[0];
     for(int i=0; i < _commands.count(); ++i) {
         if(_commands[i].object == p1.object && _commands[i].property == p1.property) {
             _commands[i].newValue = p1.newValue;
             //if(!_objects.contains(p1.object)) _objects << p1.object;
             return true;
         }
     }
     _commands << p1;
     //if(!_objects.contains(p1.object)) _objects << p1.object;
     return true;
 }
 
 class CommandNewItem: public QUndoCommand
 {
 public:
     CommandNewItem(WorldModel* worldModel, StepCore::Item* item, StepCore::ItemGroup* parent, bool create)
             : _worldModel(worldModel), _item(item), _parent(parent), _create(create), _shouldDelete(create) {
-        if(!create) findLinks(static_cast<StepCore::ItemGroup*>(_worldModel->world()));
+        if(!create) findLinks(item, static_cast<StepCore::ItemGroup*>(_worldModel->world()));
     }
     ~CommandNewItem() { if(_shouldDelete) delete _item; }
 
     void redo() Q_DECL_OVERRIDE;
     void undo() Q_DECL_OVERRIDE;
 
 protected:
-    void findLinks(StepCore::ItemGroup* group);
+    void findLinks(StepCore::Item* itemToMatch, StepCore::ItemGroup* groupToSearch);
     void removeItem();
     void readdItem();
 
     WorldModel* _worldModel;
     StepCore::Item* _item;
     StepCore::ItemGroup* _parent;
     bool _create;
     bool _shouldDelete;
 
     typedef QPair<StepCore::Object*, const StepCore::MetaProperty*> Link;
     QList<Link> _links;
 };
 
-void CommandNewItem::findLinks(StepCore::ItemGroup* group)
+void CommandNewItem::findLinks(StepCore::Item* itemToMatch, StepCore::ItemGroup* groupToSearch)
 {
-    StepCore::ItemList::const_iterator end = group->items().end();
-    for(StepCore::ItemList::const_iterator it = group->items().begin(); it != end; ++it) {
-        const StepCore::MetaObject* mobj = (*it)->metaObject();
+    StepCore::ItemGroup* itemToDeleteGroup = dynamic_cast<StepCore::ItemGroup*>(_item);
+
+    for (StepCore::Item *item : groupToSearch->items()) {
+        if (item == itemToMatch) // no need to find links in itself
+            continue;
+        if (itemToDeleteGroup && itemToDeleteGroup->contains(item)) // no need to find links if the item is a child of the item that is being deleted
+            continue;
+        const StepCore::MetaObject* mobj = item->metaObject();
         for(int i=0; i<mobj->propertyCount(); ++i) {
             if(mobj->property(i)->userTypeId() != qMetaTypeId<StepCore::Object*>()) continue;
-            if(_item == mobj->property(i)->readVariant(*it).value<StepCore::Object*>())
-                _links << qMakePair(static_cast<StepCore::Object*>(*it), mobj->property(i));
+            if(itemToMatch == mobj->property(i)->readVariant(item).value<StepCore::Object*>())
+                _links << qMakePair(static_cast<StepCore::Object*>(item), mobj->property(i));
         }
         if(mobj->inherits<StepCore::ItemGroup>())
-            findLinks(static_cast<StepCore::ItemGroup*>(*it));
+            findLinks(itemToMatch, static_cast<StepCore::ItemGroup*>(item));
+    }
+
+    // If the item being deleted is a group, be sure we also find links for all its children
+    StepCore::ItemGroup* itemToMatchGroup = dynamic_cast<StepCore::ItemGroup*>(itemToMatch);
+    if (itemToMatchGroup) {
+        for (StepCore::Item *child : itemToMatchGroup->items()) {
+            findLinks(child, groupToSearch);
+        }
     }
 }
 
 void CommandNewItem::removeItem()
 {
     foreach(const Link& link, _links)
         link.second->writeVariant(link.first, QVariant::fromValue<StepCore::Object*>(NULL));
     //qDebug("%d links removed", _links.count());
     _worldModel->removeCreatedItem(_item);
 }
 
 void CommandNewItem::readdItem()
 {
     _worldModel->addCreatedItem(_item, _parent);
     foreach(const Link& link, _links)
         link.second->writeVariant(link.first, QVariant::fromValue<StepCore::Object*>(_item));
     //qDebug("%d links restored", _links.count());
 }
 
 void CommandNewItem::redo()
 {
     if(_create) readdItem();
     else removeItem();
     _shouldDelete = !_create;
 }
 
 void CommandNewItem::undo()
 {
     if(_create) removeItem();
     else readdItem();
     _shouldDelete = _create;
 }
 
 class CommandSetSolver: public QUndoCommand
 {
 public:
     CommandSetSolver(WorldModel* worldModel, StepCore::Solver* solver)
             : _worldModel(worldModel), _solver(solver) {}
     ~CommandSetSolver() { delete _solver; }
     void redo() Q_DECL_OVERRIDE { _solver = _worldModel->swapSolver(_solver); }
     void undo() Q_DECL_OVERRIDE { _solver = _worldModel->swapSolver(_solver); }
 
 protected:
     WorldModel* _worldModel;
     StepCore::Solver* _solver;
 };
 
 class CommandSimulate: public QUndoCommand
 {
 public:
     CommandSimulate(WorldModel* worldModel);
     ~CommandSimulate() { delete _worldCopy; }
 
     void done();
 
     void redo() Q_DECL_OVERRIDE;
     void undo() Q_DECL_OVERRIDE;
 
     const QString& startTime() const { return _startTime; }
     const QString& endTime() const { return _endTime; }
 
 protected:
     typedef QPair<int, int> PairInt;
     PairInt indexToPair(const QModelIndex &index);
     QModelIndex pairToIndex(PairInt pair);
 
     WorldModel* _worldModel;
     StepCore::World* _worldCopy;
     StepCore::World* _oldWorld;
     StepCore::World* _newWorld;
 
     QString _startTime;
     QString _endTime;
 };
 
 CommandSimulate::PairInt CommandSimulate::indexToPair(const QModelIndex &index)
 {
     if(index.parent().isValid()) return PairInt(1, index.row());
     else return PairInt(0, index.row());
 }
 
 QModelIndex CommandSimulate::pairToIndex(CommandSimulate::PairInt pair)
 {
     if(pair.first == 0) {
         if(pair.second == 0) return _worldModel->worldIndex();
         else if(pair.second == 1) return _worldModel->solverIndex();
         else if(pair.second == 2) return _worldModel->collisionSolverIndex();
         else if(pair.second == 3) return _worldModel->constraintSolverIndex();
         else return QModelIndex();
     } else return _worldModel->childItemIndex(pair.second);
 }
 
 CommandSimulate::CommandSimulate(WorldModel* worldModel)
 {
     _worldModel = worldModel;
     _oldWorld = _worldCopy = _worldModel->_world;
 
     /*
     QList<PairInt> selection;
     foreach(QModelIndex index, _worldModel->_selectionModel->selection().indexes())
         selection << indexToPair(index);
     PairInt current = indexToPair(_worldModel->_selectionModel->currentIndex());
     */
 
     _worldModel->beginResetModel();
     _newWorld = _worldModel->_world = new StepCore::World(*_oldWorld);
     _worldModel->endResetModel();
 
     _startTime = _worldModel->formatProperty(_worldModel->_world, NULL,
                             _worldModel->_world->metaObject()->property(QStringLiteral("time")),
                             WorldModel::FormatEditable);
 
     /*
     foreach(PairInt pair, selection)
         _worldModel->_selectionModel->select(pairToIndex(pair), QItemSelectionModel::Select);
     _worldModel->_selectionModel->setCurrentIndex(pairToIndex(current), QItemSelectionModel::Current);
     */
 }
 
 void CommandSimulate::done()
 {
     _endTime = _worldModel->formatProperty(_worldModel->_world, NULL,
                             _worldModel->_world->metaObject()->property(QStringLiteral("time")),
                             WorldModel::FormatEditable);
 }
 
 void CommandSimulate::redo()
 {
     if(_newWorld != _worldModel->_world) {
         _worldModel->beginResetModel();
         _worldModel->_world = _newWorld;
         _worldCopy = _oldWorld;
 
         _worldModel->endResetModel();
         _worldModel->_selectionModel->setCurrentIndex(
                 _worldModel->worldIndex(), QItemSelectionModel::SelectCurrent);
     }
 }
 
 void CommandSimulate::undo()
 {
     _worldModel->beginResetModel();
     _worldModel->_world = _oldWorld;
     _worldCopy = _newWorld;
 
     _worldModel->endResetModel();
     _worldModel->_selectionModel->setCurrentIndex(
             _worldModel->worldIndex(), QItemSelectionModel::SelectCurrent);
 }
 
 WorldModel::WorldModel(QObject* parent)
     : QAbstractItemModel(parent), _actions(0)
 {
     _selectionModel = new QItemSelectionModel(this, this);
     _undoStack = new QUndoStack(this);
     _clipboard = new Clipboard(this);
     _worldFactory = new WorldFactory();
     _world = new StepCore::World();
 
     _simulationCommand = NULL;
     _simulationTimer = new QTimer(this);
     _simulationTimer0 = new QTimer(this);
     _simulationTimer0->setSingleShot(true);
     _simulationTimer0->setInterval(0);
     setSimulationFps(25); // XXX KConfig ?
 
     _simulationThread = new SimulationThread(&_world);
     _simulationThread->start();
 
     connect(_simulationTimer, &QTimer::timeout,
                 this, &WorldModel::simulationFrameBegin);
     connect(_simulationTimer0, &QTimer::timeout,
                 this, &WorldModel::simulationFrameBegin);
     connect(_simulationThread, &SimulationThread::worldEvolveDone,
                 this, &WorldModel::simulationFrameEnd, Qt::QueuedConnection);
 
     _updatingTimer = new QTimer(this);
     _updatingTimer->setSingleShot(true);
     _updatingTimer->setInterval(0);
     _updatingFullUpdate = false;
     _updatingRecalcFn = false;
 
     connect(_updatingTimer, &QTimer::timeout,
                 this, &WorldModel::doEmitChanged);
 
     resetWorld();
 
     _simulationFrameWaiting = false;
     _simulationFrameSkipped = false;
     _simulationStopping = false;
     _simulationPaused = false;
 
 }
 
 WorldModel::~WorldModel()
 {
     delete _simulationThread;
     delete _worldFactory;
     delete _world;
 }
 
 void WorldModel::resetWorld()
 {
     Q_ASSERT(!isSimulationActive());
     beginResetModel();
     if(_world->name().isEmpty()) {
         // XXX: check that loaded items has unique names !
         _world->setName(getUniqueName(QStringLiteral("world")));
     }
     if(NULL == _world->solver()) {
         _world->setSolver(new StepCore::AdaptiveEulerSolver());
         _world->solver()->setName(getUniqueName(QStringLiteral("solver")));
     }
     if(NULL == _world->collisionSolver()) {
         _world->setCollisionSolver(new StepCore::GJKCollisionSolver());
         _world->collisionSolver()->setName(getUniqueName(QStringLiteral("collisionSolver")));
     }
     if(NULL == _world->constraintSolver()) {
         _world->setConstraintSolver(new StepCore::CGConstraintSolver());
         _world->constraintSolver()->setName(getUniqueName(QStringLiteral("constraintSolver")));
     }
     _undoStack->clear();
 
     _world->doCalcFn();
     endResetModel();
 
     _selectionModel->setCurrentIndex(worldIndex(), QItemSelectionModel::SelectCurrent);
 
     emitChanged(true, false);
 }
 
 void WorldModel::emitChanged(bool fullUpdate, bool recalcFn)
 {
     //qDebug() << "emitChanged(): " << world()->time() << endl;
     if(fullUpdate) _updatingFullUpdate = true;
     if(recalcFn) _updatingRecalcFn = true;
     if(!_updatingTimer->isActive()) _updatingTimer->start(0);
 }
 
 void WorldModel::doEmitChanged()
 {
     if(_updatingRecalcFn) _world->doCalcFn();
     emit worldDataChanged(!_updatingFullUpdate);
     if(_updatingFullUpdate) {
         emit dataChanged(worldIndex(), constraintSolverIndex());
     }
 }
 
 QModelIndex WorldModel::worldIndex() const
 {
     return createIndex(0, 0, _world);
 }
 
 QModelIndex WorldModel::solverIndex() const
 {
     return createIndex(1, 0, _world->solver());
 }
 
 QModelIndex WorldModel::collisionSolverIndex() const
 {
     return createIndex(2, 0, _world->collisionSolver());
 }
 
 QModelIndex WorldModel::constraintSolverIndex() const
 {
     return createIndex(3, 0, _world->constraintSolver());
 }
 
 QModelIndex WorldModel::childItemIndex(int n, StepCore::ItemGroup* group) const
 {
     if(!group) group = _world;
     return createIndex(n, 0, group->childItem(n));
 }
 
 QModelIndex WorldModel::objectIndex(StepCore::Object* obj) const
 {
     if(obj == _world) return worldIndex();
     else if(obj == _world->solver()) return solverIndex();
     else if(obj == _world->collisionSolver()) return collisionSolverIndex();
     else if(obj == _world->constraintSolver()) return constraintSolverIndex();
     else {
         StepCore::Item* item = dynamic_cast<StepCore::Item*>(obj);
         for(StepCore::Item* it = item; it != _world; it = it->group()) {
             if(it == NULL) return QModelIndex();
         }
         STEPCORE_ASSERT_NOABORT(item && item->group());
         return createIndex(item->group()->childItemIndex(item), 0, item);
         //return itemIndex(_world->childItemIndex(dynamic_cast<const StepCore::Item*>(obj)));
     }
 }
 
 StepCore::Object* WorldModel::object(const QModelIndex& index) const
 {
     if(index.isValid()) return static_cast<StepCore::Object*>(index.internalPointer());
     else return NULL;
 }
 
 #if 0
 void WorldModel::objectChanged(const StepCore::Object* /*object*/)
 {
     if(!_updating) {
         _world->doCalcFn();
         emitChanged(false);
     }
 }
 #endif
 
 StepCore::Item* WorldModel::item(const QModelIndex& index) const
 {
     return dynamic_cast<StepCore::Item*>(object(index));
 }
 
 QVariant WorldModel::data(const QModelIndex &index, int role) const
 {
     if(!index.isValid()) return QVariant();
     StepCore::Object* obj = static_cast<StepCore::Object*>(index.internalPointer());
 
     if(role == Qt::DisplayRole) {
         return formatNameFull(obj);
     } else if(role == Qt::ToolTipRole) {
         const_cast<WorldModel*>(this)->simulationPause();
         return createToolTip(index); // XXX
     } else if(role == Qt::DecorationRole) {
         if(_worldFactory->hasObjectIcon(obj->metaObject())) 
             return _worldFactory->objectIcon(obj->metaObject());
         return QVariant();
     } else if(role == FormattedNameRole) {
         return formatName(obj);
     } else if(role == ClassNameRole) {
         return obj->metaObject()->className();
     }
 
     return QVariant();
 }
 
 QModelIndex WorldModel::index(int row, int /*column*/, const QModelIndex &parent) const
 {
     if(!parent.isValid()) {
         if(row == 0) return worldIndex();
         else if(row == 1) return solverIndex();
         else if(row == 2) return collisionSolverIndex();
         else if(row == 3) return constraintSolverIndex();
     } else {
         StepCore::ItemGroup* group = dynamic_cast<StepCore::ItemGroup*>(object(parent));
         if(group) return createIndex(row, 0, group->childItem(row));
         //if(parent.internalPointer() == _world) return itemIndex(row);
     }
     return QModelIndex();
 }
 
 QModelIndex WorldModel::parent(const QModelIndex &index) const
 {
     if(!index.isValid()) return QModelIndex();
     else if(index.internalPointer() == _world) return QModelIndex();
     else if(index.internalPointer() == _world->solver()) return QModelIndex();
     else if(index.internalPointer() == _world->collisionSolver()) return QModelIndex();
     else if(index.internalPointer() == _world->constraintSolver()) return QModelIndex();
     else {
         StepCore::Item* item = dynamic_cast<StepCore::Item*>(object(index));
         if(item && item->group()) return objectIndex(item->group());
         else return QModelIndex();
     }
 }
 
 int WorldModel::rowCount(const QModelIndex &parent) const
 {
     if(!parent.isValid()) {
         Q_ASSERT(_world->solver());
         Q_ASSERT(_world->collisionSolver());
         Q_ASSERT(_world->constraintSolver());
         return 4;
         /*
         int count = 1;
         if(_world->solver()) ++count;
         if(_world->collisionSolver()) ++count;
         return count;
         */
     }
     else if(dynamic_cast<StepCore::ItemGroup*>(object(parent)))
         return dynamic_cast<StepCore::ItemGroup*>(object(parent))->childItemCount();
     else return 0;
 }
 
 int WorldModel::columnCount(const QModelIndex& /*parent*/) const
 {
     return 1;
 }
 
 QString WorldModel::newItemName(const QString& className)
 {
     return getUniqueName(className);
 }
 
 StepCore::Item* WorldModel::createItem(const QString& className, StepCore::ItemGroup* parent)
 {
     StepCore::Item* item = _worldFactory->newItem(className);
     if(item == NULL) return NULL;
     item->setName(getUniqueName(className));
     return item;
 }
 
 StepCore::Item* WorldModel::newItem(const QString& className, StepCore::ItemGroup* parent)
 {
     StepCore::Item* item = _worldFactory->newItem(className);
     if(item == NULL) return NULL;
     item->setName(getUniqueName(className));
     pushCommand(new CommandNewItem(this, item, parent, true));
     return item;
 }
 
 void WorldModel::addItem(StepCore::Item* item, StepCore::ItemGroup* parent)
 {
     //if(item->name().isEmpty()) item->setName(getUniqueName(item->metaObject()->className()));
     pushCommand(new CommandNewItem(this, item, parent, true));
 }
 
 StepCore::Solver* WorldModel::newSolver(const QString& className)
 {
     StepCore::Solver* solver = _worldFactory->newSolver(className);
     if(solver == NULL) return NULL;
 
     // Copy similary named properties
     // XXX: is it right ?
     StepCore::Solver* oldSolver = _world->solver();
     for(int i=0; i<oldSolver->metaObject()->propertyCount(); ++i) {
         const StepCore::MetaProperty* oldProperty = oldSolver->metaObject()->property(i);
         const StepCore::MetaProperty* newProperty = solver->metaObject()->property(oldProperty->name());
         if(newProperty && newProperty->isWritable()) {
             newProperty->writeVariant(solver, oldProperty->readVariant(oldSolver));
         }
     }
 
     //solver->setName(_world->solver()->name());
     //solver->setName(getUniqueName(name)); // XXX: is it better ?
 
     pushCommand(new CommandSetSolver(this, solver));
     return solver;
 }
 
 void WorldModel::deleteItem(StepCore::Item* item)
 {
     pushCommand(new CommandNewItem(this, item, item->group(), false));
 }
 
 void WorldModel::deleteSelectedItems()
 {
     simulationPause();
 
     QList<StepCore::Item*> items;
     foreach(QModelIndex index, selectionModel()->selectedIndexes()) {
         // Do not delete world item
         if (index == worldIndex()) continue;
         
         StepCore::Item* it = item(index);
         if (it) items << it;
     }
 
     foreach(StepCore::Item* it, items) {
         for(StepCore::Item* it1 = it->group(); it1 != 0 && it1 != _world; it1 = it1->group()) {
             if(items.contains(it1)) { items.removeOne(it); break; }
         }
     }
 
     if(!items.isEmpty()) {
         beginMacro(items.count()==1 ? i18n("Delete %1", items[0]->name())
                                     : i18n("Delete several items"));
         foreach(StepCore::Item* it, items) deleteItem(it);
         endMacro();
     }
 }
 
 void WorldModel::addCreatedItem(StepCore::Item* item, StepCore::ItemGroup* parent)
 {
     if(!parent) parent = _world;
     beginInsertRows(objectIndex(parent), parent->childItemCount(), parent->childItemCount());
     parent->addItem(item);
     endInsertRows();
     emitChanged(true, true);
 }
 
 void WorldModel::removeCreatedItem(StepCore::Item* item)
 {
     STEPCORE_ASSERT_NOABORT(item->group());
     QModelIndex index = objectIndex(item);
     beginRemoveRows(index.parent(), index.row(), index.row());
     item->group()->removeItem(item);
     endRemoveRows();
     emitChanged(true, true);
 }
 
 StepCore::Solver* WorldModel::swapSolver(StepCore::Solver* solver)
 {
     bool selected = selectionModel()->isSelected(solverIndex());
     bool current = selectionModel()->currentIndex() == solverIndex();
     StepCore::Solver* oldSolver = _world->removeSolver();
     _world->setSolver(solver);
     if(selected) selectionModel()->select(solverIndex(), QItemSelectionModel::Select);
     if(current) selectionModel()->setCurrentIndex(solverIndex(), QItemSelectionModel::Current);
     emitChanged(true, true);
     return oldSolver;
 }
 
 void WorldModel::pushCommand(QUndoCommand* command)
 {
     Q_ASSERT(!_simulationFrameWaiting || _simulationPaused);
     if(!isSimulationActive()) {
         _undoStack->push(command);
     } else {
         command->redo();
         delete command;
     }
 }
 
 void WorldModel::beginMacro(const QString& text)
 {
     // XXX: simulation could be started (by hotkey) during macro creation
     if(!isSimulationActive())
         _undoStack->beginMacro(text);
 }
 
 void WorldModel::endMacro()
 {
     // XXX: simulation could be stopped during macro creation
     if(!isSimulationActive())
         _undoStack->endMacro();
 }
 
 void WorldModel::setProperty(StepCore::Object* object,
             const StepCore::MetaProperty* property, const QVariant& value, UndoFlags flags)
 {
     Q_ASSERT(object != NULL); Q_ASSERT(property != NULL);
     pushCommand(new CommandEditProperty(this, object, property, value, !flags.testFlag(UndoNoMerge)));
 }
 
 QString WorldModel::formatName(const StepCore::Object* object) const
 {
     if(!object) return i18n("<no object>");
     else if(!object->name().isEmpty()) return object->name();
     return i18n("<unnamed>");
 }
 
 QString WorldModel::formatNameFull(const StepCore::Object* object) const
 {
     if(!object) return i18n("<no object>");
     return i18n("%1: %2", formatName(object), object->metaObject()->classNameTr());
 }
 
 QString WorldModel::formatProperty(const StepCore::Object* object,
                                    const StepCore::Object* objectErrors,
                                    const StepCore::MetaProperty* property,
                                    FormatFlags flags) const
 {
     int pr = Settings::floatDisplayPrecision();
 
     QString units;
     if(!flags.testFlag(FormatHideUnits)) {
         if(!flags.testFlag(FormatEditable) && !property->units().isEmpty()) 
             units.append(" [").append(property->units()).append("]");
 #ifdef STEP_WITH_UNITSCALC
         else if(flags.testFlag(FormatEditable) && !property->units().isEmpty()
                     && property->userTypeId() == QMetaType::Double) 
             units.append(" ").append(property->units());
 #endif
     }
 
     const StepCore::MetaProperty* pv = objectErrors ?
             objectErrors->metaObject()->property(property->name() + "Variance") : NULL;
 
     // Common property types
     if(property->userTypeId() == QMetaType::Double) {
         QString error;
         if(pv) error = QStringLiteral(" ± %1")
                         .arg(sqrt(pv->readVariant(objectErrors).toDouble()), 0, 'g', pr)
                         .append(units);
         return QString::number(property->readVariant(object).toDouble(), 'g', pr).append(units).append(error);
     } else if(property->userTypeId() == qMetaTypeId<StepCore::Vector2d>()) {
         QString error;
         if(pv) {
             StepCore::Vector2d vv = pv->readVariant(objectErrors).value<StepCore::Vector2d>();
             error = QStringLiteral(" ± (%1,%2)")
                         .arg(sqrt(vv[0]), 0, 'g', pr).arg(sqrt(vv[1]), 0, 'g', pr).append(units);
         }
         StepCore::Vector2d v = property->readVariant(object).value<StepCore::Vector2d>();
         return QStringLiteral("(%1,%2)").arg(v[0], 0, 'g', pr).arg(v[1], 0, 'g', pr).append(units).append(error);
     } else if(property->userTypeId() == qMetaTypeId<StepCore::Vector2dList >() ) {
         // XXX: add error information
 //         if(pv) qDebug() << "Unhandled property variance type" << endl;
         StepCore::Vector2dList list =
                 property->readVariant(object).value<StepCore::Vector2dList >();
         QString string;
         unsigned int end = flags.testFlag(FormatEditable) ? list.size() : qMin<unsigned int>(10, list.size());
         for(unsigned int i=0; i<end; ++i) {
             if(!string.isEmpty()) string += ',';
             string += QStringLiteral("(%1,%2)").arg(list[i][0], 0, 'g', pr)
                                             .arg(list[i][1], 0, 'g', pr);
         }
         if(!flags.testFlag(FormatEditable) && end != 0 && end < list.size()) string += QLatin1String(",...");
         string.append(units);
         return string;
     } else {
         // default type
         // XXX: add error information
         //if(pe) error = QString::fromUtf8(" ± ").append(pe->readString(_objectErrors)).append(units);
         //if(pv) qDebug() << "Unhandled property variance type" << endl;
         Q_ASSERT(!pv);
         QString str = property->readString(object);
         if(!flags.testFlag(FormatEditable) && str.length() > 50)
             str = str.left(50).append("...");
         return str.append(units);
     }
 }
 
 QString WorldModel::createToolTip(const QModelIndex& index) const
 {
     //Q_ASSERT(object != NULL);
     Q_ASSERT(index.isValid());
     QString toolTip = i18n("<nobr><h4><u>%1</u></h4></nobr>", index.data(Qt::DisplayRole).toString());
     toolTip += QLatin1String("<table>");
 
     StepCore::Object* object = this->object(index);
     StepCore::Item* item = dynamic_cast<StepCore::Item*>(object);
     const StepCore::Object* objectErrors = NULL;
     if(item) {
         if(world()->errorsCalculation()) objectErrors = item->objectErrors();
         else objectErrors = item->tryGetObjectErrors();
     }
 
     for(int i=0; i<object->metaObject()->propertyCount(); ++i) {
         const StepCore::MetaProperty* p = object->metaObject()->property(i);
         /*QString units;
         if(!p->units().isEmpty())
             units.append(" [").append(p->units()).append("]");
         QString value = p->readString(object);
         if(p->userTypeId() == qMetaTypeId<StepCore::Vector2dList >()) {
             // XXX: don't use readString in this case !
             value.replace("),(", QString(")%1<br />(").arg(units));
             value.append(units);
             if(value.count("<br />") > 10) {
                 value = value.section("<br />", 0, 9);
                 value.append("<br />...");
             }
         } else {
             value.append(units);
         }*/
         toolTip += i18n("<tr><td>%1&nbsp;&nbsp;</td><td>%2</td></tr>", p->nameTr(),
                             formatProperty(object, objectErrors, p));
     }
     toolTip += QLatin1String("</table>");
     //qDebug("%s", toolTip.toAscii().constData());
     return toolTip;
 }
 
 QMenu* WorldModel::createContextMenu(const QModelIndex& index)
 {
     //Q_ASSERT(object != NULL);
     Q_ASSERT(index.isValid());
     QMenu* menu = new QMenu();
     menu->addSection(index.data(Qt::DisplayRole).toString());
     ItemMenuHandler* handler = _worldFactory->newItemMenuHandler(object(index), this, menu);
     handler->populateMenu(menu, _actions);
     return menu;
 }
 
 void WorldModel::clearWorld()
 {
     _world->clear();
     resetWorld();
 }
 
 bool WorldModel::saveXml(QIODevice* device)
 {
     StepCore::XmlFile file(device);
 
     if(file.save(_world)) {
         return true;
     } else {
         _errorString = file.errorString();
         return false;
     }
 }
 
 bool WorldModel::loadXml(QIODevice* device)
 {
     _world->clear();
     StepCore::XmlFile file(device);
     bool ret = file.load(_world, _worldFactory);
     if(!ret) { _world->clear(); _errorString = file.errorString(); }
     resetWorld();
     return ret;
 }
 
 bool WorldModel::checkUniqueName(const QString& name) const
 {
     if(name.isEmpty()) return false;
     if(_world->object(name) != NULL) return false;
     return true;
 }
 
 QString WorldModel::getUniqueName(const QString& className) const
 {
     QString classNameCopy = className;
     classNameCopy[0] = classNameCopy[0].toLower();
     for(int n=1; ; ++n) {
         QString name = classNameCopy + QString::number(n);
         if(checkUniqueName(name)) return name;
     }
     return QString();
 }
 
 void WorldModel::simulationPause()
 {
     if(!_simulationFrameWaiting || _simulationPaused) return;
 
     // Try to lock but do not wait mode than one frame
     if(_simulationThread->mutex()->tryLock(1000/_simulationFps)) {
         // We still need to setAbortEvolve(true) since
         // _simulationThread->doWorldEvolve() could be called before
         _world->setEvolveAbort(true);
     } else {
         //qDebug() << "simulationPause: simulation aborted" << endl;
         Q_ASSERT(isSimulationActive());
         Q_ASSERT(_simulationCommand);
         Q_ASSERT(_simulationFrameWaiting);
         //Q_ASSERT(!_simulationStopping);
 
         _world->setEvolveAbort(true);
 
         // Mutex will be locked as soon as current frame is aborted
         _simulationThread->mutex()->lock();
     }
     // We can release mutex just now since new simulation frame can
     // only be started from this thread when control returns to event loop
     _simulationThread->mutex()->unlock();
     _simulationPaused = true;
 
     //qDebug() << "simulationPause!" << endl;
     // XXX: do we need to reset evolveAbort here (and add threadAbort var) ?
     // XXX: do we need to call emitChanged here ?
 }
 
 void WorldModel::setSimulationFps(int simulationFps)
 {
     _simulationFps = simulationFps;
     _simulationTimer->setInterval(1000/simulationFps);
 }
 
 bool WorldModel::isSimulationActive()
 {
     return _simulationTimer->isActive();
 }
 
 void WorldModel::simulationStart()
 {
     Q_ASSERT(!isSimulationActive());
     Q_ASSERT(!_simulationCommand);
 
     //_undoStack->beginMacro(i18n("Simulate"));
     _simulationCommand = new CommandSimulate(this);
     _world->setEvolveAbort(false);
     _simulationFrameWaiting = false;
     _simulationFrameSkipped = false;
     _simulationStopping = false;
     _simulationPaused = false;
 
     _simulationFrames = 0;
     _simulationStartTime = QTime::currentTime();
     _simulationTimer->start();
 
     //emitChanged();
 }
 
 void WorldModel::simulationStop()
 {
     Q_ASSERT(isSimulationActive());
     Q_ASSERT(_simulationCommand);
     _simulationStopping = true;
     if(_simulationFrameWaiting) {
         simulationPause();
     } else {
         simulationFrameEnd(0);
     }
 }
 
 void WorldModel::simulationFrameBegin()
 {
     Q_ASSERT(isSimulationActive());
     Q_ASSERT(_simulationCommand);
     Q_ASSERT(!_simulationStopping);
 
     //qDebug() << "simulationFrameBegin(): " << world()->time() << endl;
 
     if(_simulationFrameWaiting) { // TODO: warn user
         _simulationFrameSkipped = true;
         return;
     }
 
     if(_updatingTimer->isActive()) {
         // Wait for updating
         _simulationTimer0->start();
         return;
     }
 
     //qDebug("emit simulationDoFrame() t=%#x", int(QThread::currentThread()));
     //qDebug() << "simulationFrameBegin" << endl;
     _simulationFrameWaiting = true;
     _simulationPaused = false;
     _simulationThread->doWorldEvolve(1.0/_simulationFps);
     //qDebug("emitted simulationDoFrame()");
 }
 
 void WorldModel::simulationFrameEnd(int result)
 {
     Q_ASSERT(isSimulationActive());
     Q_ASSERT(_simulationCommand);
     Q_ASSERT(_simulationFrameWaiting || _simulationStopping);
 
     ++_simulationFrames;
     if(_simulationFrames == 50) {
         qDebug("FPS: %f", double(_simulationFrames) /
                     _simulationStartTime.msecsTo(QTime::currentTime()) * 1000);
         _simulationStartTime = QTime::currentTime();
         _simulationFrames = 0;
     }
 
     //qDebug() << "simulationFrameEnd" << endl;
 
     // It's OK to be aborted
     if(result == StepCore::Solver::Aborted) {
         //qDebug("simulation frame aborted!");
         result = StepCore::Solver::OK;
     }
 
     // If current frame was aborted we can resume it now
     _world->setEvolveAbort(false);
 
     // Update GUI
     _simulationFrameWaiting = false;
     emitChanged(false, false);
 
     // Stop if requested or simulation error occurred
     if(_simulationStopping || result != StepCore::Solver::OK) {
         _simulationCommand->done();
         _undoStack->beginMacro(i18n("Simulate %1 → %2",
                 _simulationCommand->startTime(), _simulationCommand->endTime()));
         _undoStack->push(_simulationCommand);
         _undoStack->endMacro();
         _simulationCommand = NULL;
 
         _simulationTimer->stop();
         _simulationTimer0->stop();
         emit simulationStopped(result);
         return;
     }
 
     if(_simulationFrameSkipped) {
         _simulationFrameSkipped = false;
         _simulationTimer0->start();
         //QApplication::processEvents(); // XXX
         //if(isSimulationActive() && !_simulationFrameWaiting)
         //    simulationFrameBegin();
     }
 }
 
 QList<StepCore::Item*> WorldModel::selectedItems()
 {
     QList<StepCore::Item*> items;
     foreach (QModelIndex index, selectionModel()->selectedIndexes()) {
         // Do not delete world item
         if (index == worldIndex()) continue;
         
         StepCore::Item* it = item(index);
         if (it) items << it;
     }
     
     foreach (StepCore::Item* it, items) {
         for (StepCore::Item* it1 = it->group(); it1 != 0 && it1 != _world; it1 = it1->group()) {
             if (items.contains(it1)) {
                 items.removeOne(it);
                 break;
             }
         }
     }
     
     return items;
 }
 
 void WorldModel::cutSelectedItems()
 {
     simulationPause();
     
     QList<StepCore::Item*> items = selectedItems();
     
     _clipboard->copy(items);
     
     if (!items.isEmpty()) {
         beginMacro(items.count() == 1 ? i18n("Cut %1", items[0]->name()) :
                    i18n("Cut several items"));
         foreach (StepCore::Item* it, items) deleteItem(it);
         endMacro();
     }
 }
 
 void WorldModel::copySelectedItems()
 {
     simulationPause();
     
     QList<StepCore::Item*> items = selectedItems();
     
     _clipboard->copy(items);
 }
 
 void WorldModel::pasteItems()
 {
     simulationPause();
     
     QList<StepCore::Item*> items = _clipboard->paste(_worldFactory);
     if (items.isEmpty()) return;
     
     beginMacro(items.count() == 1 ? i18n("Pasted %1", items[0]->name()) :
                i18n("Pasted several items"));
     QItemSelection selection;
     foreach (StepCore::Item* item, items) {
         QString className = item->metaObject()->className();
         item->setName(getUniqueName(className));
         addItem(item, _world);
         QModelIndex index = objectIndex(item);
         selection.select(index, index);
     }
     if (!selection.isEmpty()) {
         selectionModel()->select(selection, QItemSelectionModel::ClearAndSelect);
     }
     endMacro();
 }
diff --git a/stepcore/itemgroup.h b/stepcore/itemgroup.h
index 99122df..ed31c72 100644
--- a/stepcore/itemgroup.h
+++ b/stepcore/itemgroup.h
@@ -1,104 +1,106 @@
 /* This file is part of StepCore library.
    Copyright (C) 2007 Vladimir Kuznetsov <ks.vladimir@gmail.com>
    Copyright (C) 2014 Inge Wallin        <inge@lysator.liu.se>
 
    StepCore library is free software; you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
    the Free Software Foundation; either version 2 of the License, or
    (at your option) any later version.
 
    StepCore library is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
    GNU General Public License for more details.
 
    You should have received a copy of the GNU General Public License
    along with StepCore; if not, write to the Free Software
    Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301  USA
 */
 
 /** \file itemgroup.h
  *  \brief Contains the ItemGroup object.
  */
 
 #ifndef STEPCORE_ITEMGROUP_H
 #define STEPCORE_ITEMGROUP_H
 
 
 #include <vector> // XXX: Replace if Qt is enabled.
 
 #include "types.h"
 #include "item.h"
 
 
 namespace StepCore
 {
 
 
 /** \ingroup world
  *  \brief Groups several items together
  */
 class ItemGroup : public Item
 {
     STEPCORE_OBJECT(ItemGroup)
 
 public:
     /** Constructs empty group */
     explicit ItemGroup(const QString& name = QString()) : Item(name) {}
     /** Constructs a copy of the group (deep copy) */
     ItemGroup(const ItemGroup& group);
     /** Destroys the group and all its subitems */
     ~ItemGroup();
 
     /** Assignment operator (deep copy)
      *  \warning Do not call this on groups already attached to the world */
     ItemGroup& operator=(const ItemGroup& group);
 
     /** Get list of all direct child items in the ItemGroup */
     const ItemList& items() const  { return _items; }
 
+    bool contains(const Item* item) const { return std::find(_items.begin(), _items.end(), item) != _items.end(); }
+
     /** Get list of all items in the ItemGroup
      *  \note This operation takes long time since it
      *        recursively traverses all child groups */
     ItemList allItems() const { ItemList l; allItems(&l); return l; }
     /** Get list of all items in the ItemGroup
      *  \param items Array to store items
      *  \note This operation takes long time since it
      *        recursively traverses all child groups */
     void allItems(ItemList* items) const;
 
     /** Add new item to the group */
     virtual void addItem(Item* item);
     /** Remove item from the group (you should delete item yourself) */
     virtual void removeItem(Item* item);
     /** Delete item from the group (it actually deletes item) */
     virtual void deleteItem(Item* item) { removeItem(item); delete item; }
 
     /** Deletes all items */
     void clear();
 
     /** Finds direct child item in items() */
     int  childItemIndex(const Item* item) const;
     /** Get direct child count */
     int  childItemCount() const { return _items.size(); }
     /** Get direct child item by its index */
     Item* childItem(int index) const { return _items[index]; }
     /** Get direct child item by its name */
     Item* childItem(const QString& name) const;
     /** Get any descendant item by its name */
     Item* item(const QString& name) const;
 
     /** Recursively call setWorld for all children objects */
     void setWorld(World* world) Q_DECL_OVERRIDE;
     /** Recursively call worldItemRemoved for all children objects */
     void worldItemRemoved(Item* item) Q_DECL_OVERRIDE;
     
 private:
     ItemList  _items;
 };
 
 
 } // namespace StepCore
 
 
 #endif