T Node
   String value
   Node? left
   Node? right

   F (value, Node? left = N, Node? right = N)
      .value = String(value)
      .left = left
      .right = right

   F tree_indent() -> [String]
      V tr = I .right != N {.right.tree_indent()} E [‘-- (null)’]
      R [‘--’(.value)] [+] (I .left != N {.left.tree_indent()} E [‘-- (null)’]).map(a -> ‘  |’a)
                       [+] [‘  `’tr[0]] + tr[1..].map(a -> ‘   ’a)

V tree = Node(1, Node(2, Node(4, Node(7)), Node(5)), Node(3, Node(6, Node(8), Node(9))))
print(tree.tree_indent().join("\n"))