<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _callable_once(func):
 """ Returns a function that is only callable once; any other call will do nothing """


    def once(*args, **kwargs):
        if not once.called:
            once.called = True
            return func(*args, **kwargs)

    once.called = False
    return once