docs: def instantiate(self, cls=None):
          """Return an instantiated Expression as
          ``cls.create(*self.args, **self.kwargs)``
  
          Args:
              cls (class): The class of the instantiated expression. If not
              given, ``self.cls`` will be used.
          """
          if cls is None:
              cls = self.cls
          if cls is None:
              raise TypeError("cls must a class")
          return cls.create(*self.args, **self.kwargs)