docs: function sign(custom, request) {
      /**
       * An Epoch timestamp is required for the digital signature.
       */
      let epoch = Util.epochInMilliseconds();
      /**
       * Override the generated Epoch timestamp if a timestamp was included in the
       * custom headers.
       */
      if (custom.WMSecurity && custom.WMSecurity.Timestamp) {
          epoch = custom.WMSecurity.Timestamp;
      }
      /**
       * The generated digital signature.
       */
      let signature = digitalSignature(custom, request, epoch);
      /**
       * Override the generated digital signature if a signature was included in the
       * custom headers.
       */
      if (custom.WMSecurity && custom.WMSecurity.AuthSignature) {
          signature = custom.WMSecurity.AuthSignature;
      }
      /**
       * The signed request headers according to Walmart API spec.
       */
      let signedHeaders = {
          'WM_SVC.NAME': custom.WMService.Name,
          'WM_QOS.CORRELATION_ID': custom.WMQOS.CorrelationId,
          'WM_SEC.TIMESTAMP': epoch,
          'WM_SEC.AUTH_SIGNATURE': signature,
          'WM_CONSUMER.CHANNEL.TYPE': custom.WMConsumer.Channel.Type,
          'WM_CONSUMER.ID': custom.WMConsumer.ConsumerId,
          'Accept': custom.Accept,
          'Content-Type': custom.Accept
      };
      return signedHeaders;
  }