CombineWithPrevious
} else if (edit->CombineWithPrevious(top.Get())) {
virtual bool CombineWithPrevious(
virtual bool CombineWithPrevious(const Command* previous);
} else if (command->CombineWithPrevious(top)) {
virtual bool CombineWithPrevious(const Command* previous);
} else if (command->CombineWithPrevious(top)) {