why does phobos use [0, 5, 8, 9][] instead of [0, 5, 8, 9] in examples?

[Timothee Cour via Digitalmars-d-learn]

Eg, code like this in std.algorithm:
assert(equal(setSymmetricDifference(a, b), [0, 5, 8, 9][]));
why not just:
assert(equal(setSymmetricDifference(a, b), [0, 5, 8, 9]));
?