code: def p_Yield(p):
      '''
      Yield : YIELD
            | YIELD Expression
            | YIELD Expression COMMA Expression
      '''
      if len(p) == 2:
          p[0] = Yield(None, None)
      elif len(p) == 3:
          p[0] = Yield(p[2], None)
      else:
          p[0] = Yield(p[2], p[4]f p_Yield(p):
      '''
      Yield : YIELD
            | YIELD Expression
            | YIELD Expression COMMA Expression
      '''
      if len(p) == 2:
          p[0] = Yield(None, None)
      elif len(p) == 3:
          p[0] = Yield(p[2], None)
      else:
          p[0] = Yield(p[2], p[4])
docs: Yield : YIELD
            | YIELD Expression
            | YIELD Expression COMMA Expression