module DescendantsTracker

def self.extended(descendant)
  setup(descendant)
end

def self.setup(descendant)
  descendant.instance_variable_set('@descendants', [])
end

def add_descendant(descendant)
  ancestor = superclass
  if ancestor.respond_to?(:add_descendant)
    ancestor.add_descendant(descendant)
  end
  descendants.unshift(descendant)
  self
end

def descendants
  @descendants
end

def inherited(descendant)
  super
  DescendantsTracker.setup(descendant)
  add_descendant(descendant)
end