docs: def kwargs(self):
          """The dictionary of keyword-only arguments for the instantiation of
          the Expression"""
          # Subclasses must override this property if and only if they define
          # keyword-only arguments in their __init__ method
          if hasattr(self, '_has_kwargs') and self._has_kwargs:
              raise NotImplementedError(
                  "Class %s does not provide a kwargs property"
                  % str(self.__class__.__name__))
          return {}