code: def AddOperands(self, lhs, rhs):
      
      if isinstance(lhs, Expression) and isinstance(rhs, Expression):
        self.args = [lhs, rhs]
      else:
        raise errors.ParseError(
            'Expected expression, got {0:s} {1:s} {2:s}'.format(
                lhs, self.operator, rhs))
docs: Add an operand.