 Solution

The return types suggest an immutable sequence implementation: insertion and deletion return a new Sequence[A], while the receiver can remain unchanged. A mutable implementation would more naturally return Unit for in-place edits, though it could return itself. Thus the signatures are evidence of an intended immutable object implementation, not a logical guarantee. For the snapshot solution below, require that returned and retained Sequence values really are immutable. The owning SlideSuite can still have a mutable variable referring to its current immutable sequence.