public static <T> Stream<? extends T> walkOptional(
            T first, 
            Function<T, Optional<T>> traverser,
            WalkingOrder order) {
        
        requireNonNulls(first, traverser, order);
        return walkOptional(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