def lit(a):
    def inner(s):
        if s.startswith(a):
            return a, s[len(a):]
        return None, s
    return inner

def seq(*exprs):
    def inner(s):
        results = []
        new_s = s
        for expr in exprs:
            result, new_s = expr(new_s)
            if result is None:
                return None, s
            results.append(result)
        return results, new_s
    return inner

def alt(*exprs):
    def inner(s):
        for expr in exprs:
            result, new_s = expr(s)
            if result is not None:
                return result, new_s
        return None, s
    return inner

def opt(expr):
    def inner(s):
        result, s = expr(s)
        if result is None:
            return '', s
        return result, s
    return inner

def many(expr):
    def inner(s):
        results = []
        while True:
            result, new_s = expr(s)
            if result is None:
                break
            results.append(result)
            s = new_s
        return results, s
    return inner

def many1(expr):
    def inner(s):
        result, s = many(expr)(s)
        if not result:
            return None, s
        return result, s
    return inner