As I understand the [following snippet](https://github.com/realworldocaml/examples/blob/32ea926861a0b728813a29b0e4cf20dd15eb486e/code/imperative-programming/value_restriction-13.rawscript) taken from [chapter describing value restriction](https://realworldocaml.org/v1/en/html/imperative-programming-1.html#side-effects-and-weak-polymorphism) supposed to show off the use of `+'a t` annotation, which is missing.