Looks like the comment is zero-indexed as well. I wouldn't consider that a typo.
3 4 5 100 200 300 400 7 8 9 10
Weird, I tried it on http://johnearnest.github.io/ok/index.html and that agrees with the doc.. kinda confusing. Seems like it should be
?[x;3;y] / insert y at position 3
?[x;3 4;y] / replace what's in position 3 by y