treeRec xs  = Fork (treeRec ys) (treeRec zs)
  where  n = length xs `div` 2
         (ys,zs) = splitAt n xs
treeRec xs  = Fork (treeRec ys) (treeRec zs)
  where  n = length xs `div` 2
         (ys,zs) = splitAt n xs
treeRec xs  = Fork (treeRec ys) (treeRec zs)
  where  n = length xs `div` 2
         (ys,zs) = splitAt n xs