unification bug

[kowey]

The code

main =
 do print $ head $ unify left right
    print $ head $ unify right left
 where
  left  = map (GVar . show) [1..3]
  right = drop 1 left ++ [GConst ["X"]]

The output

([?1,?1,X],fromList [("1",X),("2",X),("3",X)])
([?2,?3,X],fromList [("1",X),("2",X),("3",X)])

I think it should be

The output

([X,X,X],fromList [("1",X),("2",X),("3",X)])
([X,X,X],fromList [("1",X),("2",X),("3",X)])

The fix is probably simple: just replace after unify