code: public boolean exists(final AbstractHtml tag)
              throws NullValueException, InvalidTagException {
          if (tag == null) {
              throw new NullValueException("tag cannot be null");
          }
          if (NoTag.class.isAssignableFrom(tag.getClass())) {
              throw new InvalidTagException(
                      "classes like NoTag.class cannot be used to find tags as it's not a logical tag in behaviour.");
          }
          return browserPage.contains(tag);
      }
docs: Checks the existence of a tag instance.
  
  @param tag
  @return true if the given tag instance exists anywhere in the browser
  page.
  @throws NullValueException
  if the tag is null
  @throws InvalidTagException
  if the given tag is {@code NoTag}.
  @since 2.1.8
  @author WFF