code: def requires(self, from_, if_=None, **data):
          
          return self.transition(from_, None, if_, **data)
docs: Decorates a method that may be called if the given state is currently active.
          Registers a transition internally, but does not change the state.
  
          :param from_: Required state to allow this call (can be a state group)
          :param if_: Validator(s) that, given the object, must all return True for the call to proceed
          :param data: Additional metadata, stored on the `StateTransition` object as a :attr:`data` attribute