# File lib/declarative_authorization/obligation_scope.rb, line 128
    def add_obligation_condition_for( path, expression )
      raise "invalid expression #{expression.inspect}" unless expression.is_a?( Array ) && expression.length == 3
      add_obligation_join_for( path )
      obligation_conditions[@current_obligation] ||= {}
      ( obligation_conditions[@current_obligation][path] ||= Set.new ) << expression
    end