code: public void assignNestedDrivers(XEvent event,
  			Map<List<String>, String> drivers) {
  		XCostDriver.instance().assignNestedValues(event, drivers);
  	}
docs: Assigns (to the given event) multiple cost drivers given their key lists.
  The i-th element in the key list should correspond to an i-level
  attribute with the prescribed key. Note that as a side effect this method
  creates attributes when it does not find an attribute with the proper
  key.
  
  @see #assignNestedAmounts(XEvent, Map)
  
  @param event
  Event to assign the cost drivers to.
  @param drivers
  Mapping from key lists to cost drivers which are to be
  assigned.