public void testConcurrent() {
ElementHandlerRegistry r1 = ElementHandlerRegistry.ROOT.createExtension();
ElementHandlerRegistry r2 = r1.createExtension();
// Check overriding in the child registry with concurrent propagation
r2.registerRenderer("x", a);
r2.registerRenderer("x", b);
r1.registerRenderer("x", c);
r1.registerRenderer("x", d);
assertSame(b, r2.getRenderer(el1));