import Data.List( delete, nub, sort )

-- Remove the first part of xs from ys and then recurse. 
difference (x:xs) ys = difference xs (delete x ys)
-- Once the list is empty we can just use y.
difference [] (y:_) = y

-- Get the count of some element x in xs.
count xs x = length [x' | x' <- xs, x' == x]

-- Return a list of all the frequencies for unique elements of xs.
frequencies xs = map (count xs) us where us = nub xs

-- If the frequency lists have the same elements,
-- they can be replaced. i.e., they are isomorphic.
isomorphic xs ys = sort (frequencies xs) == sort (frequencies ys)