Compute "substring" implies nonnegative argument check

[barockobamo]

From #26 :

There is one place where we compute a "substring" in the code;
need to check whether the argument is always nonnegative.

[IgnoredAmbience]

"the code" is unclear, jsref or js_of_ast? The motivation for this issue may be hard to determine.

[barockobamo]

I can't tell for the same reason as in #49 . @brabalan ? Do you know from where it comes ?

[brabalan]

I think this is js_of_ast. I vaguely remember something in the standard library.