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When a playlist is deleted and drops_references(), any undo/redo StatefulDiffCommand referncing playlist invoke Destructible::drop_references() of the Command. This leads to command_death(). As opposed to UndoTransaction::clear() the StatefulDiffCommand was not destroyed. In case of playlists StatefulDiffCommand::_changes contains PBD::SequenceProperty<std::list<boost::shared_ptr<Region> > > and shared pointer reference of the playlist regions were kept indefinitely. This fixes the following scenario: New session, import an file, delete the created track, clean up unused sources (delete unused playlists)[, quit]. A reference to the imported region was kept, because of the playlist's undo command (insert region). Yet the source file was deleted. PS. Most playlist changes are accompanied by GUI zoom/selection MementoCommands. Those are currently never directly dropped. command_death() leaves those in place.
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