This module defines signatures that are to be included in other signatures to ensure a consistent interface to invariant-style functions. There is a signature (S, S1, S2, S3) for each arity of type. Usage looks like:
      type t
      include Invariant.S with type t := t


      type 'a t
      include Invariant.S1 with type 'a t := 'a t
type 'a t = 'a -> unit
type 'a inv = 'a t
module type S = sig .. end
type t
val invariant : t inv
module type S1 = sig .. end
type 'a t
val invariant : 'a inv -> 'a t inv
module type S2 = sig .. end
type ('a, 'b) t
val invariant : 'a inv -> 'b inv -> ('a, 'b) t inv
module type S3 = sig .. end
type ('a, 'b, 'c) t
val invariant : 'a inv -> 'b inv -> 'c inv -> ('a, 'b, 'c) t inv
module type Invariant = sig .. end
include sig .. end with type __pa_nonrec_0 := 'a0 t
type 'a0 __pa_nonrec_0 = 'a0 t
type 'a t = 'a __pa_nonrec_0
module type S = S
module type S1 = S1
module type S2 = S2
module type S3 = S3
val invariant : Source_code_position0.t ->
'a -> ('a -> Sexplib.Sexp.t) -> (unit -> unit) -> unit
invariant here t sexp_of_t f runs f (), and if f raises, wraps the exception in an Error.t that states "invariant failed" and includes both the exception raised by f, as well as sexp_of_t t. Idiomatic usage looks like:
        invariant _here_ t <:sexp_of< t >> (fun () ->
          ... check t's invariants ... )
val check_field : 'a -> 'b t -> ('a, 'b) Fieldslib.Field.t -> unit
check_field is used when checking invariants using Fields.iter. It wraps an exception raised when checking a field with the field's name. Idiomatic usage looks like:
        type t =
          { foo : Foo.t;
            bar : Bar.t;
        with fields

        let invariant t : unit =
          invariant "Foo.invariant" t <:sexp_of< t >> (fun () ->
            let check f = Invariant.check_field t f in
              ~foo:(check Foo.invariant)
              ~bar:(check Bar.invariant))