def rev_priv_hierarchy
      if @rev_priv_hierarchy.nil?
        @rev_priv_hierarchy = {}
        privilege_hierarchy.each do |key, value|
          value.each do |val| 
            @rev_priv_hierarchy[val] ||= []
            @rev_priv_hierarchy[val] << key
          end
        end
      end
      @rev_priv_hierarchy
    end
def rev_priv_hierarchy
      if @rev_priv_hierarchy.nil?
        @rev_priv_hierarchy = {}
        privilege_hierarchy.each do |key, value|
          value.each do |val| 
            @rev_priv_hierarchy[val] ||= []
            @rev_priv_hierarchy[val] << key
          end
        end
      end
      @rev_priv_hierarchy
    end
{[priv, ctx] => [priv, ...]}