When using WithExpectation it should most often be matched by a ClearExpectation.
Not calling ClearExpectation can cause the expectation to appear in the failure message of subsequent assertions inside an AssertionScope.

To make AssertCollectionsHaveSameItems return a chainable ContinuationOfGiven<T> I made CollectionWithIndex<T>.
It might not be very elegant, but it was the only way I could come up with.