code: void checkLabel(final Label label, final boolean checkVisited,
              final String msg) {
          if (label == null) {
              throw new IllegalArgumentException("Invalid " + msg
                      + " (must not be null)");
          }
          if (checkVisited && labels.get(label) == null) {
              throw new IllegalArgumentException("Invalid " + msg
                      + " (must be visited first)");
          }
      }
docs: Checks that the given label is not null. This method can also check that
  the label has been visited.
  
  @param label
  the label to be checked.
  @param checkVisited
  <tt>true</tt> to check that the label has been visited.
  @param msg
  a message to be used in case of error.