How to append or prepend on a Scala mutable.Seq
Solution 1:
Mutability for sequences only guarantees that you'll be able to swap out the items for different ones (via the update method), as you can with e.g. primitive arrays. It does not guarantee that you'll be able to make the sequence larger (that's what the Growable trait is for) or smaller (Shrinkable).
Buffer is the abstract trait that contains Growable and Shrinkable, not Seq.