class Result(object): pass
class Failed(Result): pass
failed = Failed()
def cached(f, cacheCount=[0]):
    attr = "t" + str(cacheCount[0]); cacheCount[0] += 1
    name = f.__name__
    class CacheResult(Result):
        def __init__(self, i, rv): setattr(self, attr, (i, rv))
    cache = {}
    def deco(self, i):
        key = i
        if key in cache and cache[key] is failed: raise ParseError()
        elif key in cache: return getattr(cache[key], attr)
        cache[key] = failed
        i, rv = f(self, i)
        cache[key] = CacheResult(i, rv)
        self.lastMatch.append((name, i))
        return i, rv
    deco.__name__ = name
    return deco