История изменений

first revert . head . filter (atExit) $ go lab empty initial
  where 
    go _ _ [] = error "no exit"
    go lab prev (z@(x,hist):zs) = 
       let z' = next x 
                 & filter (canEnter lab)
                 & filter (`S.notElem` prev)
                 & map (\y -> (y, t:hist))
       in z:go lab (prev `S.union` (fromList z')) (zs++z')
       where canEnter = {- put your implementation here -} undefined
             next     = {- put your implementation here -} undefined

first revert . head . filter (atExit) $ go lab empty initial
  where 
    go _ _ [] = error "no exit"
    go lab prev (z@(x,hist):zs) = 
       let z' = next x 
                 & filter (canEnter lab)
                 & filter (`S.notElem` prev)
                 & map (\y -> (y, t:hist))
       in z:go lab (prev `S.union` (fromList z')) (zs++z')
       where canEnter = {- put your implementation here -} undefined
             next     = {- put your implementation here -} undefined

только непонятно, что за агенты, какие агенты, кому они нужны...