code: public void assignNestedDrivers(XTrace trace,
  			Map<List<String>, String> drivers) {
  		XCostDriver.instance().assignNestedValues(trace, drivers);
  	}
docs: Assigns (to the given trace) 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(XTrace, Map)
  
  @param trace
  Trace to assign the cost drivers to.
  @param drivers
  Mapping from key lists to cost drivers which are to be
  assigned.