How to use the docopt.Required function in docopt

def parse_expr(tokens, options):
    """expr ::= seq ( '|' seq )* ;"""
    seq = parse_seq(tokens, options)
    if tokens.current() != '|':
        return seq
    result = [Required(*seq)] if len(seq) > 1 else seq
    while tokens.current() == '|':
        tokens.move()
        seq = parse_seq(tokens, options)
        result += [Required(*seq)] if len(seq) > 1 else seq
    return [Either(*result)] if len(result) > 1 else result

def parse_expr(tokens, options):
    """expr ::= seq ( '|' seq )* ;"""
    seq = parse_seq(tokens, options)
    if tokens.current() != '|':
        return seq
    result = [Required(*seq)] if len(seq) > 1 else seq
    while tokens.current() == '|':
        tokens.move()
        seq = parse_seq(tokens, options)
        result += [Required(*seq)] if len(seq) > 1 else seq
    return [Either(*result)] if len(result) > 1 else result

def parse_atom(tokens, options):
    """atom ::= '(' expr ')' | '[' expr ']' | 'options'
             | long | shorts | argument | command ;
    """
    token = tokens.current()
    result = []
    if token in '([':
        tokens.move()
        matching, pattern = {'(': [')', Required], '[': [']', Optional]}[token]
        result = pattern(*parse_expr(tokens, options))
        if tokens.move() != matching:
            raise tokens.error("unmatched '%s'" % token)
        return [result]
    elif token == 'options':
        tokens.move()
        return [OptionsShortcut()]
    elif token.startswith('--') and token != '--':
        return parse_long(tokens, options)
    elif token.startswith('-') and token not in ('-', '--'):
        return parse_shorts(tokens, options)
    elif token.startswith('<') and token.endswith('>') or token.isupper():
        return [Argument(tokens.move())]
    else:
        return [Command(tokens.move())]