I wonder how many readers reacted on the fact that your statement is making the invalid assumption that "any sequence" is multi-pass, breaking the (current) semantic requirements of Sequence.
This is a very common mistake (that I myself use(d) to make). After I realized this, I started this thread which might be an Interesting read.
EDIT: No, I was wrong, your subscript will work as expected even with a single-pass sequence.