docs: public final boolean skipDoubleColon() throws IOException
      {
          int c = skip_over_whitespace();
          if (c != ':') {
              unread_char(c);
              return false;
          }
          c = read_char();
          if (c != ':') {
              unread_char(c);
              unread_char(':');
              return false;
          }
          return true;
      }