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  data Tree a = Leaf | Branch (Tree a) a (Tree a)
  tree = Branch (Branch Leaf 1 Leaf) 3 (Branch Leaf 2 Leaf)

  type Leaf = { 
    type: 'leaf' 
  }
  type Branch<a> = {
    type: 'branch'
    v: a
    l: Tree<a>
    r: Tree<a>
  }
  type Tree<a> = Leaf | Branch<a>
  const isLeaf = (l: any): l is Leaf => l?.type === 'leaf'
  const isBranch = <a,>(b: any): b is Branch<a> => b?.type === 'branch'
  const isTree = <a,>(t: any): t is Tree<a> => isLeaf(t) || isBranch<a>(t)
  const Leaf = (): Leaf => ({ type: 'leaf' })
  const Branch = <a,>(v: a, l: Tree<a> = Leaf(), r: Tree<a> = Leaf()): Branch<a> => ({ type: 'branch', v, l, r })
  const tree = Branch(2, Branch(1), Branch(3))