addAll() is folded wrongly when index is used

[sabi0]

In this particular case (when index = 0) the best folding is probably this:
vCharSequence = sequence + vCharSequence;

In case the index is different from 0 I don't think it can be folded without loosing semantics.
Unless you are willing to bother with slices like:
vCharSequence = vCharSequence[:3] + sequence + vCharSequence[3:]