QUndoStack: How can I cancel in QUndoCommand::redo() the QUndoStack::push() function?

Code:

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Hi!
 
I am calling QUndoStack::push( myCommand )
In myCommand::redo() I am calling a function called "bool dosomething()" 
If the result is false I want to cancel the push()-Function. 
 
If this is not possible, how can I remove explicitly the last QUndoCommand from QUndoStack?
 
Sorry for my broken english...
 
Mike from Vienna

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