code: public static void requireEmptyText(Node node, String errMsg)
              throws IllegalArgumentException {
          require((node instanceof Text) || (node instanceof Comment),
                  errMsg + ": " + node.toString());
          if (node instanceof Text) {
              Text text = (Text)node;
              String textValue = text.getData();
              require(textValue.trim().length() == 0, errMsg + ": " + textValue);
          }
      }
docs: Assert that the given org.w3c.doc.Node is a comment element or a Text element and
  that it ontains whitespace only, otherwise throw an IllegalArgumentException using
  the given error message. This is helpful when nothing is expected at a certain
  place in a DOM tree, yet comments or whitespace text nodes can appear.
  
  @param node                      A DOM Node object.
  @param errMsg                    String used to in the IllegalArgumentException
  constructor if thrown.
  @throws IllegalArgumentException If the expression is false.