* This is final - override {@link #appendContents(TreeFormatter)}  instead to add type specific content.
     */
    @Override
    public final TreeFormatter describeContents(TreeFormatter formatter) {
        formatter.node("collection type: ").appendType(getClass()).append(" (id: ").append(String.valueOf(System.identityHashCode(this))).append(")");
        formatter.startChildren();
        appendContents(formatter);
        formatter.endChildren();

        if (!lenient) {
            resolutionHost.rethrowFailure("files", collectingVisitor.getFailures());
        }
    }

    @Override
    protected void appendContents(TreeFormatter formatter) {
        formatter.node("contains: " + getDisplayName());
    }

    SelectedArtifactSet getSelectedArtifacts() {
        if (selectedArtifacts == null) {

    }

    @Override
    public FileCollectionInternal filter(final Spec<? super File> filterSpec) {
        return new FilteredFileCollection(this, filterSpec) {
            @Override
            protected void appendContents(TreeFormatter formatter) {
                formatter.node("filtered collection");
                formatter.startChildren();
                CompositeFileCollection.this.describeContents(formatter);

        this.patternSupplier = patternSupplier;
    }

    @Override
    public String getDisplayName() {
        return tree.getDisplayName();
    }

    @Override
    protected void appendContents(TreeFormatter formatter) {
        formatter.node("backing tree");
        formatter.startChildren();
        tree.describeContents(formatter);
        formatter.endChildren();
    }

        this.source = ImmutableSet.copyOf(source);
    }

    @Override
    public String getDisplayName() {
        return "file collection";
    }

    @Override
    protected void appendContents(TreeFormatter formatter) {
        formatter.node("source");
        formatter.startChildren();
        for (FileCollectionInternal files : source) {
            files.describeContents(formatter);
        }

    public MinimalFileTree getTree() {
        return tree;
    }

    @Override
    public String getDisplayName() {
        return tree.getDisplayName();
    }

    @Override
    protected void appendContents(TreeFormatter formatter) {
        formatter.node("tree: " + tree);
    }

    @Override
    public void visitDependencies(TaskDependencyResolveContext context) {
        if (tree instanceof Buildable) {

    }

    @Override
    public String getDisplayName() {
        return outputDirectories.toString();
    }

    @Override
    protected void appendContents(TreeFormatter formatter) {
        formatter.node("source set: " + outputDirectories.toString());
        formatter.node("output directories");
        formatter.startChildren();

        super(taskDependencyFactory, patternSetFactory);
        this.collection = collection;
    }

    @Override
    protected void appendContents(TreeFormatter formatter) {
        formatter.node("backing collection");
        formatter.startChildren();
        collection.describeContents(formatter);
        formatter.endChildren();
    }

            nested.add(source);
        }

        @Override
        protected void appendContents(TreeFormatter formatter) {
            formatter.node("source");
            formatter.startChildren();
            appendItem(formatter, source);
            formatter.endChildren();
        }

    }

    @Override
    public String getDisplayName() {
        return displayName == null ? "file collection" : displayName;
    }

    @Override
    protected void appendContents(TreeFormatter formatter) {
        if (displayName != null) {
            formatter.node("display name: " + displayName);
        }
        List<Object> paths = new ArrayList<>();
        value.collectSource(paths);