code: public void startDocument() throws IOException {
          ResponseWriter writer = getWrapped();
          String encoding = writer.getCharacterEncoding( );
          if( encoding == null ) {
              encoding = "utf-8";
          }
          writer.writePreamble("<?xml version='1.0' encoding='" + encoding + "'?>\n");
          writer.startElement("partial-response", null);
          FacesContext ctx = FacesContext.getCurrentInstance();
          if (null != ctx && null != ctx.getViewRoot()) {
              String id = ctx.getViewRoot().getContainerClientId(ctx);
              writer.writeAttribute("id", id, "id");
          }
      }
docs: <p class="changed_added_2_0">Write the start of a partial response.</p>
  
  @throws IOException if an input/output error occurs
  @since 2.0