class ViewModel::AccessControl::Tree

def include_from(ancestor)
  unless ancestor < ViewModel::AccessControl::Tree
    raise ArgumentError.new("Invalid ancestor: #{ancestor}")
  end
  @included_checkers << ancestor
  self::AlwaysPolicy.include_from(ancestor::AlwaysPolicy)
  ancestor.view_policies.each do |view_name, ancestor_policy|
    policy = find_or_create_policy(view_name)
    policy.include_from(ancestor_policy)
  end
end