docs: public static void addMention(Stanza stanza, int begin, int end, BareJid jid) {
          URI uri;
          try {
              uri = new URI("xmpp:" + jid.toString());
          } catch (URISyntaxException e) {
              throw new AssertionError("Cannot create URI from bareJid.");
          }
          ReferenceElement reference = new ReferenceElement(begin, end, ReferenceElement.Type.mention, null, uri);
          stanza.addExtension(reference);
      }