This type is not documented, it should especially since it exposes a tuple rather than a record. Something Like 

```ocaml
(** The type for backtrace slots. 
  {ul 
  {- [Known_location (is_raise, filename, line, schar, echar)] is for known 
     location where [is_raise] is [true] if ....}
  {- [Unknown_location is_raise] is for unknown location where [is_raise] 
     is [true if] ... }}
```