docs: def output(self, stream, value):
          """SPL output port assignment expression.
  
          Arguments:
              stream(Stream): Output stream the assignment is for.
              value(str): SPL expression used for an output assignment. This can be a string, a constant, or an :py:class:`Expression`.
  
          Returns:
              Expression: Output assignment expression that is valid as a the context of this operator.
          """
          if stream not in self.outputs:
              raise ValueError("Stream is not an output of this operator.")
          e = self.expression(value)
          e._stream = stream
          return e