ParamList :
              | Param
              | ParamList COMMA Param

def p_ParamList(p):
    '''
    ParamList :
              | Param
              | ParamList COMMA Param
    '''
    if len(p) == 1:
        p[0] = ParamList(None, None)
    elif len(p) == 2:
        p[0] = ParamList(None, p[1])
    else:
        p[0] = ParamList(p[1], p[3])