* 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();

            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);