Example of RELAX NG schema for XRC with a custom element.

frob.rnc

default namespace = "http://www.wxwidgets.org/wxxrc"
namespace xrc = "http://www.wxwidgets.org/wxxrc"

include "http://www.wxwidgets.org/wxxrc" {
  customClasses =
    element object {
        attribute class { "Frobnicator" } &
        stdObjectNodeAttributes &
        stdWindowProperties &
        [xrc:p="o"] element num_times {_, t_integer }*
    }
}