def fib_(n):
   if n in [0,1]: return lambda a:a, 1, lambda:None
   else: return ( (lambda a,b: b),
                  None,
                  (lambda: ( lambda a,b: a+b,
                             combine(fib_(n-1)) + combine(fib_(n-2)))
      )
     )
 
 def combine(comb, n, next):
   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
 
 
 def fib(n):
   comb, n, next = fib_(n)
   return combine(comb, n, next)