[Caml-list] strange type error with -principal

[Alexey Egorov <alex.only.d@gmail.com>]

Hello,

I'm getting very confusing error when compiling with -principal:

> Error: This expression has type int op
>       but an expression was expected of type 'a
>       This instance of int is ambiguous:
>       it would escape the scope of its equation

What is the "instance of int"?

Here is the code - https://pastebin.com/T74haahx
I'm mostly confused by the fact that changing pattern from (op, idx)
to simple value binding eliminates this error.

Thanks!