code: function middlewareReducer (callSequence, middleware) {
      return function nextHandler (err) {
        // if the previous middleware passed an error argument
        if (err !== undefined) {
          if (isErrorHandler(middleware)) {
            // call the current middleware if it is an error handler middleware
            middleware(err, req, res, callSequence)
          } else {
            // else skip the current middleware and call the intermediate sequence
            callSequence(err)
          }
        } else {
          // if no error argument is passed
          if (isErrorHandler(middleware)) {
            // skip the current middleware if it is an errorHandler
            callSequence()
          } else {
            // else call it
            middleware(req, res, callSequence)
          }
        }
      }
    }
docs: Reduce the middleware sequence to a nested middleware handler sequence
  @function
  @inner
  @private
  @param {Function} callSequence intermediate resulting call sequence
  @param {Function} middleware the current middleware
  @returns {Function} the new intermediate resulting call sequence