{-# LANGUAGE DataKinds        #-}
{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE KindSignatures   #-}
{-# LANGUAGE TypeFamilies     #-}
 
data Tree = Leaf | Fork Tree Tree
 
type family Branch (t :: Tree) (u :: Tree) :: Tree where
    Branch Leaf       Leaf       = Leaf
    Branch Leaf       (Fork l r) = Fork (Branch l r) (Branch r l)
    Branch (Fork l r) u          = Fork (Branch u l) (Branch u r)
 
type Four = Fork (Fork (Fork Leaf Leaf) (Fork Leaf Leaf)) (Fork (Fork Leaf Leaf) (Fork Leaf Leaf))
 
class SmthTree (t :: Tree)
instance SmthTree Leaf
instance (SmthTree l, SmthTree r) => SmthTree (Fork l r)
 
test :: SmthTree (Branch (Branch (Branch Four Four) (Branch Four Four)) (Branch (Branch Four Four) (Branch Four Four))) => ()
test = ()
 
main = print ()

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