Hey guys,

I propose to extend UndoableCommandInterface from CommandInterface for the following reasons:

1. It contains only undo method which doesn't look like command at all
2. We can't correctly work this interface. Please look at these examples with simplified invokers:

function executeCommand(CommandInterface $command)
{
    $command->execute();
    $command->undo(); // undo is not part of the CommandInterface
}

function executeUndoableCommand(UndoableCommandInterface $command)
{
    $command->execute(); // execute is not part of UndoableCommandInterface
    $command->undo();
}

In both cases we can't follow the declared interface to work with the command.

Thanks,
Ihor