opIndex, opSlice, length for joiner?

[Maverick Chardet via Digitalmars-d]

And coming back to the specialized version of walkLength: in the 
case where joiner does not implement save, should I still allow 
the specialized version? (and in this case do what: consume the 
range, make a copy of "this" and hope for the best, something 
else?)

Maverick Chardet