mergeBy :: (a -> a -> Ordering) -> [a] -> [a] -> [a]
mergeBy cmp [] ys = ys
mergeBy cmp xs [] = xs
mergeBy cmp (x : xs) (y : ys)
  | cmp x y == GT = y : mergeBy cmp (x : xs) ys
  | cmp x y == LT = x : mergeBy cmp xs (y : ys)
  | otherwise = x : y : mergeBy cmp xs ys

instance PQueue Heap where
  toPQueue cmp xs = foldr insert (Heap cmp Nil) xs
  fromPQueue (Heap cmp Nil) = []
  fromPQueue (Heap cmp (Node i t1 a t2)) = a : mergeBy cmp (fromPQueue (Heap cmp t1)) (fromPQueue (Heap cmp t1))