> But a MutableSequence can represents a sequence that doesn't
> fit in memory. It can provide an interface to a linear 
> on-disk store. In this case creating an on-memory copy 
> is not possible.

This case is likely not worth worrying about.  If a user created such a sequence AND wrote "s += s", it is unclear whether any particular unpleasant outcome (including the current behavior) could be avoided.