Call_provers.analyze_result defaults to unknown

I found (by modifying alt-ergo configuration to add an incorrect switch) that Call_provers.analyze_result returns Unknown if no regex matches the prover output. This seems incorrect to me, I thought that Unknown meant that the prover returned Unknown explicitly. Instead it should return Failure or even HighFailure, which is also the documentation:

type prover_answer =
  | Valid
      (** The task is valid according to the prover *)
  | Invalid
      (** The task is invalid *)
  | Timeout
      (** the task timeouts, ie it takes more time than specified *)
  | OutOfMemory
      (** the task runs out of memory *)
  | StepLimitExceeded
      (** the task required more steps than the limit provided *)
  | Unknown of string
      (** The prover can't determine if the task is valid *)
  | Failure of string
      (** The prover reports a failure *)
  | HighFailure
      (** An error occured during the call to the prover or none
          of the given regexps match the output of the prover *)

What do you think?