DSL to build an enum hierarchy in a visually elegant way

Foo

package rubber.duck;

import com.google.common.collect.ImmutableSet;

public enum Foo implements FooHierarchy.Node {
    ONE,
    TWO,
    THREE,
    FOUR,
    FIVE,
    SIX,
    SEVEN;


    @Override
    public FooHierarchy.Node withChildren(FooHierarchy.Node... children) {
        return new FooHierarchy.NodeWithChildren(this, ImmutableSet.copyOf(children));
    }


    private static final FooHierarchy.Node hierarchy =
            ONE.withChildren(
                    TWO.withChildren(
                            THREE,
                            FOUR.withChildren(
                                    FIVE
                            )
                    ),
                    SIX,
                    SEVEN
            );

}

FooHierarchy

package rubber.duck;

import com.google.common.collect.ImmutableSet;

import javax.annotation.Nonnull;

public class FooHierarchy {

    public interface Node {

        Node withChildren(Node... children);

    }

    public static class NodeWithChildren implements Node {

        @Nonnull
        final Node element;
        @Nonnull
        final ImmutableSet<Node> children;

        NodeWithChildren(@Nonnull Node element, @Nonnull ImmutableSet<Node> children) {
            this.element = element;
            this.children = children;
        }

        public Node withChildren(Node... children) {
            throw new UnsupportedOperationException("Cannot add children to a node that already has children");
        }

    }

}