Fast access to the last index used

[Chat-Wane]

When inserting a new element within the sequence, the function get(index) is called twice to get the identifier of the previous and the next elements. The main target of this data structure is the collaborative editing of document. In this case, the previously inserted element are likely to be reused for the next allocation of identifier.

For instance, let us consider the sequence "QWER". Inserting a character "T" at the end will access to the identifier of "R" and the maximal bound. Inserting again at the end will access to the identifier of the previously inserted "T" and again, the maximal bound...