Yield : YIELD
          | YIELD Expression
          | YIELD Expression COMMA Expression

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])