File file2 = new File("f2");
File file3 = new File("f3");
TestFileCollection collection1 = new TestFileCollection(file1, file2);
TestFileCollection collection2 = new TestFileCollection(file2, file3);
FileCollection sum = collection1.plus(collection2);
assertThat(sum, instanceOf(UnionFileCollection.class));
assertThat(sum.getFiles(), equalTo(toLinkedSet(file1, file2, file3)));
}