Classes inheriting from RWSequenceable have an innate ordering. You can speak meaningfully of their first or last object, or of their 6th or ith object.

These classes are implemented generally as either a vector, or a singly-linked or doubly-linked list. Vector-based classes make good stacks and queues, but poor insertions in the middle. If you exceed the capacity of a vector-based collection class, it will automatically resize, but it may exact a significant performance penalty to do so.

Note that the binary and B-tree classes can be considered sequenceable in the sense that they are sorted, and therefore have an innate ordering. However, their ordering is determined internally by the relative value of the collected objects, rather than by an insertion order. In other words, you cannot arbitrarily insert an object into a sorted collection in any position you want because it might not remain sorted. Hence, these classes are subclassed separately.