* id (or name depending on the level and version). Null if it doesn't
* exist.
*/
public UnitDefinition getUnitDefinition(String id) {
UnitDefinition unitDefinition = getListOfUnitDefinitions().firstHit(
new NameFilter(id));
// Checking if it is not one of the predefined default units.
if (unitDefinition == null) {
unitDefinition = getPredefinedUnitDefinition(id);
}