then:
        1 * listener.onDiscovered(graph.root)
        1 * listener.onDiscovered(a)
        1 * listener.onDiscovered(b)
        0 * listener.onDiscovered(_)

        when:
        graph.add(c)
        graph.add(d)

        then:
        1 * listener.onDiscovered(c)
        1 * listener.onDiscovered(d)
        0 * listener.onDiscovered(_)
    }

     * if the node matches the criteria specified by this listener.
     */
    public abstract void onDiscovered(ModelNodeInternal node);

        }
    }

    private void maybeNotify(ModelNodeInternal node, ModelListener listener) {
        if (!node.isAtLeast(ModelNode.State.Discovered)) {
            return;
        }
        listener.onDiscovered(node);
    }

    @Nullable
    public ModelNodeInternal find(ModelPath path) {
        return flattened.get(path);
    }

    public ModelNodeInternal get(ModelPath path) {

            @Override
            public String toString() {
                return "configure matching " + spec + " using " + action.getDescriptor();
            }

            @Override
            public void onDiscovered(ModelNodeInternal node) {
                if (node.canBeViewedAs(subjectType) && spec.matches(node)) {
                    bind(ModelReference.of(node.getPath(), subjectType), role, action);
                }
            }