void canAddCollectionsTogether() {
        File file1 = new File("f1")
        File file2 = new File("f2")
        File file3 = new File("f3")
        TestFileCollection collection1 = new TestFileCollection(file1, file2)
        TestFileCollection collection2 = new TestFileCollection(file2, file3)

        when:
        FileCollection sum = collection1.plus(collection2)

        then:

        def source1 = new TestFileCollection()
        def source2 = new TestFileCollection()
        def collection = new TestCompositeFileCollection(source1, source2)

        expect:
        collection.isEmpty()
    }

    def "isn't empty when any source collection is not empty"() {
        def source1 = new TestFileCollection()
        def source2 = new TestFileCollection(file2)

        new UnionFileCollection(TestFiles.taskDependencyFactory(), source)
    }

    def containsUnionOfAllSourceCollections() {
        def source1 = new TestFileCollection(file1, file2)
        def source2 = new TestFileCollection(file2, file3)

        expect:
        def collection = newUnionFileCollection(source1, source2)
        collection.files.toList() == [file1, file2, file3]