[Haskell-cafe] I really don't understand this

[Jean-Marc Alliot]

I apologize if my question is stupid, but here is a simple Haskell 
program which never stops.

However, if I comment the line (which is apparently useless):
v<-acc
The program works like a charm...

I am using GHC 8.0.2 (debian sid) and multiset 0.3.3. If anyone has an 
idea,  I would gladly hear it.

Thanks in advance

PS: don't try to understand what the program is doing; it is just the 
reduction to a few lines of a much larger code; I have tried to find a 
smaller subset which is not working "properly".
  PPS: The program can also be downloaded from:
http://www.alliot.fr/tmp/example.hs

import qualified Data.IntMultiSet as IMS

b :: IMS.IntMultiSet
b = IMS.fromList ([1, 2, 3, 4])

search :: IMS.IntMultiSet -> Int -> IO Bool
search mynumbers nb = ins mynumbers (return False)
   where
     ins numbers acc = do
       v <- acc
       IMS.fold
         (\x acc1 ->
            let numbers2 = IMS.delete x numbers
             in IMS.fold
                  (\y acc2 ->
                     let numbers3 = IMS.delete y numbers2
                         res = x + y
                      in if res == nb
                           then (return True)
                           else ins (IMS.insert res numbers3) acc2)
                  acc1
                  numbers2)
         acc
         numbers

main = do
   v <- search b 999999999
   print v