ArgList :
            | Arg
            | ArgList COMMA Arg

def p_ArgList(p):
    '''
    ArgList :
            | Arg
            | ArgList COMMA Arg
    '''
    if len(p) == 1:
        p[0] = ArgList(None, None)
    elif len(p) == 2:
        p[0] = ArgList(None, p[1])
    else:
        p[0] = ArgList(p[1], p[3])