docs: @Nonnull
    public XServletSettings setXFrameOptions (@Nullable final EXFrameOptionType eType, @Nullable final ISimpleURL aDomain)
    {
      if (eType != null && eType.isURLRequired ())
        ValueEnforcer.notNull (aDomain, "Domain");
  
      m_eXFrameOptionsType = eType;
      m_aXFrameOptionsDomain = aDomain;
      return this;
    }