code: public static boolean exists(String url, String markup, String selector) {
          ensureMarkup(url, markup);
          Document document = documents.get(url);
          Elements elements = document.select(selector);
          return elements.size() > 0;
      }
docs: Checks if the {@code selector} identifies an element from the {@code markup}. The {@code url} is used only for caching purposes,
  to avoid parsing multiple times the markup returned for the same resource.
  
  @param url      the url that identifies the markup
  @param markup   the markup
  @param selector the selector used for retrieval
  @return {@code true} if the element identified by the {@code selector} exists, {@code false} otherwise