code: protected void checkIdRefs()
          throws XMLStreamException
      {
          /* 02-Oct-2004, TSa: Now we can also check that all id references
           *    pointed to ids that actually are defined
           */
          if (mIdMap != null) {
              ElementId ref = mIdMap.getFirstUndefined();
              if (ref != null) { // problem!
                  reportValidationProblem("Undefined id '"+ref.getId()
                                          +"': referenced from element <"
                                          +ref.getElemName()+">, attribute '"
                                          +ref.getAttrName()+"'",
                                          ref.getLocation());
              }
          }
      }
docs: /*
  /////////////////////////////////////
  Internal methods
  /////////////////////////////////////