code: public void assignDrivers(XTrace trace, Map<String, String> drivers) {
  		XCostDriver.instance().assignValues(trace, drivers);
  	}
docs: Assigns (to the given trace) multiple cost drivers given their keys. Note
  that as a side effect this method creates attributes when it does not
  find an attribute with the proper key.
  
  @see #assignAmounts(XTrace, Map)
  
  @param trace
  Trace to assign the cost drivers to.
  @param drivers
  Mapping from keys to cost drivers which are to be assigned.