docs: def calls(self, call):
          """Redefine a call.
  
          The fake method will execute your function.  I.E.::
  
              >>> f = Fake().provides('hello').calls(lambda: 'Why, hello there')
              >>> f.hello()
              'Why, hello there'
  
          """
          exp = self._get_current_call()
          exp.call_replacement = call
          return self