def rev_role_hierarchy  
      if @rev_role_hierarchy.nil?
        @rev_role_hierarchy = {}
        role_hierarchy.each do |higher_role, lower_roles|
          lower_roles.each do |role|
            (@rev_role_hierarchy[role] ||= []) << higher_role
          end
        end
      end
      @rev_role_hierarchy
    end
def rev_role_hierarchy  
      if @rev_role_hierarchy.nil?
        @rev_role_hierarchy = {}
        role_hierarchy.each do |higher_role, lower_roles|
          lower_roles.each do |role|
            (@rev_role_hierarchy[role] ||= []) << higher_role
          end
        end
      end
      @rev_role_hierarchy
    end
{[priv, ctx] => [priv, ...]}