node.cr

class Node
  include Enumerable(self)

  property left, right, data

  def initialize(@data)
  end

  def each(&block : Node ->)
    left.try &.each(&block)
    block.call(self)
    right.try &.each(&block)
  end
end

one = Node.new 1

two = Node.new 2
two.left = one

# Works
two.each do |node|
  p node
end

# Doesn't work :-(
# two.each_with_index do |node, i|
#   p 1
# end