docs: @Nonnull
    public final UnifiedResponse setETag (@Nonnull @Nonempty final String sETag)
    {
      ValueEnforcer.notEmpty (sETag, "ETag");
      ValueEnforcer.isTrue (sETag.startsWith ("\"") || sETag.startsWith ("W/\""),
                            () -> "Etag must start with a '\"' character or with 'W/\"': " + sETag);
      ValueEnforcer.isTrue (sETag.endsWith ("\""), () -> "Etag must end with a '\"' character: " + sETag);
      if (m_eHttpMethod != EHttpMethod.GET && m_eHttpMethod != EHttpMethod.HEAD)
        logWarn ("Setting an ETag on a non-GET/HEAD request may have no impact!");
  
      m_aResponseHeaderMap.setHeader (CHttpHeader.ETAG, sETag);
      return this;
    }