}

    def canAddFileTree() {
        FileTreeInternal set1 = Mock(FileTreeInternal)
        when:
        set.addToUnion(set1)

        then:
        set.sourceCollections == [set1]
    }

    def cannotAddFileCollection() {
        when:
        set.addToUnion(Mock(FileCollection))

        then:
        def e = thrown(UnsupportedOperationException)

    protected void visitChildren(Consumer<FileCollectionInternal> visitor) {
        for (FileTreeInternal sourceTree : sourceTrees) {
            visitor.accept(sourceTree);
        }
    }

    public void addToUnion(FileCollection source) {
        if (!(source instanceof FileTree)) {
            throw new UnsupportedOperationException(String.format("Can only add FileTree instances to %s.", getDisplayName()));
        }