@Override
			protected String defaultResult() {
				return "default result";
			}
This test verifies that {@link AbstractParseTreeVisitor#shouldVisitNextChild} is called before visiting the first
child. It also verifies that {@link AbstractParseTreeVisitor#defaultResult} provides the default return value for
visiting a tree.
runtime-testsuite/test/org/antlr/v4/test/runtime/java/api/TestVisitors.java