The array of the relationship between a child and his parents is output by the hierarchical display.

tree.rb

#!/usr/bin/env ruby
class Tree
  def initialize
    @data = Array.new
  end

  def <<(ary)
    @data << ary
  end

  def to_s
    root_nodes = @data.find_all { |elm| elm[1] == nil }
    to_s_children(root_nodes)
  end

  private
  def to_s_children(nodes, depth=0, indent='  ')
    return nodes if nodes.size.zero?
    nodes.sort_by{|x| x[0].to_s}.each do |node|
      puts "#{indent*depth}#{node[0]}"
      children = @data.find_all { |elm| elm[1] == node[0] }
      to_s_children(children, depth+1)
    end
  end
end

if __FILE__ == $0
  tree = Tree.new
  Module.constants.each do |elm|
    const = RUBY_VERSION < "1.9" ? eval(elm) : eval(elm.to_s)
    tree << [const, const.superclass] if const.respond_to? :superclass
  end

  puts tree
end