Truth
module True : sig ... end
module False : sig ... end
type true_ = True.t
type false_ = False.t
type ('witness, 'b) t =
| True : 'witness -> ( 'witness, True.t ) t
| False : ( 'witness, False.t ) t
type 'witness true_t = ( 'witness, true_ ) t
type 'witness false_t = ( 'witness, false_ ) t