docs: @Nonnull
    public final UnifiedResponse setXFrameOptions (@Nonnull final EXFrameOptionType eType,
                                                   @Nullable final ISimpleURL aDomain)
    {
      ValueEnforcer.notNull (eType, "Type");
      if (eType.isURLRequired ())
        ValueEnforcer.notNull (aDomain, "Domain");
  
      if (eType.isURLRequired ())
        setCustomResponseHeader (CHttpHeader.X_FRAME_OPTIONS,
                                 eType.getID () + " " + aDomain.getAsStringWithEncodedParameters ());
      else
        setCustomResponseHeader (CHttpHeader.X_FRAME_OPTIONS, eType.getID ());
      return this;
    }