docs: @Override
      public Event doExecute(final RequestContext requestContext) {
          val tgtId = WebUtils.getTicketGrantingTicketId(requestContext);
          if (StringUtils.isBlank(tgtId)) {
              return new Event(this, CasWebflowConstants.TRANSITION_ID_TGT_NOT_EXISTS);
          }
          try {
              val ticket = this.centralAuthenticationService.getTicket(tgtId, Ticket.class);
              if (ticket != null && !ticket.isExpired()) {
                  return new Event(this, CasWebflowConstants.TRANSITION_ID_TGT_VALID);
              }
          } catch (final AbstractTicketException e) {
              LOGGER.trace("Could not retrieve ticket id [{}] from registry.", e.getMessage());
          }
          return new Event(this, CasWebflowConstants.TRANSITION_ID_TGT_INVALID);
      }