[justinsaccount]

Pretty sure the output should be

  3 4 5 100 200 300 400 7 8 9 10

Otherwise that is not replacing, that is inserting.

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

[tmcb]

I see. In that case, the output is right and the comment is misleading.

[justinsaccount]

yeah, I updated my comment after finding http://johnearnest.github.io/ok/index.html so I could test this out and came to the same conclusion.