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Expand Up @@ -161,6 +161,33 @@ purpose formats are more suited to get the relevant information, than what can
be output automatically by the generic pretty-printer used by the trace
mechanism.

Print debugging is assisted by compiler builtins that allow a printed error
message to refer to the program location that the error was raised from.
For example,
```ocaml
match Message.unpack response with
| Some y -> y
| None -> Printf.printf "Invalid message at %s" __LOC__; flush stdout;
raise Invalid_argument
```
The compiler builtin `__LOC__` is substituted at compile time with the
location it occurs in the program, described as a string
"File %S, line %d, characters %d-%d".
One can also get the file name, line number, start character and
end character directly through the __POS__ builtin:
```ocaml
match Message.unpack response with
| Some y -> y
| None ->
let fname, lnum, _cstart, _cend = __POS__ in
Printf.printf "At line %d in file %s, an incorrect response was passed to Message.unpack"
lnum fname;
flush stdout; raise Invalid_argument
```

Documentation for all debugging compiler builtins is in the
[standard library documentation](https://ocaml.org/manual/5.2/api/Stdlib.html#1_Debugging).

## The OCaml Debugger

We now give a quick tutorial for the OCaml debugger (`ocamldebug`). Before
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