The library's logger.

Control the log level for Log.

qid_tag is used in log reports to tag the question (or answer) ID in the call.

pp_exn is like Fmt.exn, but pretty-prints Invariant_broken.

failf msg raises Failure msg.

• deprecated Use Fmt.failwith instead

invariant_broken msg raises Invariant_broken msg.

Unique object IDs, for debugging.