public static <T> Stream<? extends T> walk(
            T first,
            UnaryOperator<T> traverser,
            WalkingOrder order) {
        
        requireNonNulls(first, traverser, order);
        return walk(first, traverser, order, Stream.builder()).build();
    }
The walking order of the tree.
runtime-parent/runtime-config/src/main/java/com/speedment/runtime/config/internal/util/Trees.java