import functools
 
 def memoize(func):
     _cache = {}
     class NULL:pass
     NULL = NULL()
     @functools.wraps(func)
     def _inner(*args, **kwargs):
         _kwargs = tuple(
             (k, tuple(v) if hasattr(v, '__iter__') else v) for k,v in kwargs
         )
         value = _cache.get((args, _kwargs), NULL)
         if value is NULL:
             value = func(*args, **kwargs)
             _cache[(args,_kwargs)] = value
         return value
     _inner.__enter__ = lambda *_: _inner
     _inner.__exit__ = lambda *_: _cache.clear()
     _inner.reset = _cache.clear
     _inner.cache = _cache
     _inner.orig = func
     return _inner
 
 class return_if(object):
     def __init__(self, map):
         self._map = dict(
             (tuple(k) if hasattr(k, '__iter__') else (k,),v) for k,v in map.iteritems()
         )
         self._func = lambda x:x
     def _inner(self, *args):
         if args in self._map: return self._map[args]
         else: return self._func(*args)
     def __call__(self, func):
         self._func = func
         return self._inner
 
 
 @memoize
 @return_if({1: 1})
 def fact(n):
     return n*fact(n-1)
 
 
 @return_if({(1,1): 2, (2,2): 3, (3,3):4})
 def sum(a,b):
     return a+b
 
 
 @memoize
 @return_if({1: 1,2: 1})
 def fib(n):
     if n < 1: raise ValueError, "not n >= 1"
     else:
         return fib(n-1) + fib(n-2)