code: void addToWaitList(final Reference ref) {
          final URI file = ref.filename;
          assert file.isAbsolute() && file.getFragment() == null;
          if (doneList.contains(file) || waitList.contains(ref) || file.equals(currentFile)) {
              return;
          }
  
          waitList.add(ref);
      }
docs: Add the given file the wait list if it has not been parsed.
  
  @param ref reference to absolute system path