@piecewise_function
 def fact_(n):
   return ( lambda a,b: a*b,
            n,
            lambda: fact_(n-1) )
 
 @fact_.add_piece(lambda n: n==1)
 def fact_(n):
   return ( lambda a:a,
            n,
            lambda: None )
 
 def fact(n):
   comb, n, next = fact_(n)
   a = [(comb, n)]
   while next() != None:
     comb, n, next = next()
     a.insert(0, (comb,n))
 
   val = a.pop(0)
   val = val[0](val[1])
   for f, v in a:
     val = f(v, val)
   return val