code: @Override
      public CompletionStage<Result> apply(final Function<Http.RequestHeader, CompletionStage<Result>> next,
                                           final Http.RequestHeader requestHeader)
      {
          final HandlerDef handlerDef = requestHeader.attrs().get(Router.Attrs.HANDLER_DEF);
          final List<String> deadboltModifierTags = handlerDef.getModifiers().stream().filter(mt -> mt != null && mt.startsWith("deadbolt:")).collect(Collectors.toList());
          return processModifierTags(deadboltModifierTags, 0, requestHeader, next);
      }
docs: If a constraint is defined for a given route, test that constraint before allowing the request to proceed.
  
  @param next          the next step in the filter chain
  @param requestHeader the request header
  @return a future for the result