add mode.crossing.top and bot

Author: riaqn

typing/mode.ml

@@ -3064,6 +3064,10 @@ module Crossing = struct
 
     let le (t0 : t) (t1 : t) =
       match t0, t1 with Join_const c0, Join_const c1 -> Mode.Const.le c1 c0
+
+    let top : t = Join_const Mode.Const.min
+
+    let bot : t = Join_const Mode.Const.max
   end
 
   module Comonadic = struct
@@ -3089,6 +3093,10 @@ module Crossing = struct
 
     let le (t0 : t) (t1 : t) =
       match t0, t1 with Meet_const c0, Meet_const c1 -> Mode.Const.le c0 c1
+
+    let top : t = Meet_const Mode.Const.max
+
+    let bot : t = Meet_const Mode.Const.min
   end
 
   type t = (Monadic.t, Comonadic.t) monadic_comonadic
@@ -3145,6 +3153,10 @@ module Crossing = struct
   let le t0 t1 =
     Monadic.le t0.monadic t1.monadic && Comonadic.le t0.comonadic t1.comonadic
 
+  let top = { monadic = Monadic.top; comonadic = Comonadic.top }
+
+  let bot = { monadic = Monadic.bot; comonadic = Comonadic.bot }
+
   let print ppf t =
     let print_atom ppf = function
       | Modality.Atom (ax, Join_with c) -> C.print (Value.proj_obj ax) ppf c

typing/mode_intf.mli

@@ -734,6 +734,12 @@ module type S = sig
     (** [le t0 t1] returns [true] if [t0] allows more mode crossing than [t1]. *)
     val le : t -> t -> bool
 
+    (** The trivial mode crossing that crosses nothing. *)
+    val top : t
+
+    (** The mode crossing that crosses everything. *)
+    val bot : t
+
     (** Print the mode crossing by axis. Omit axes that do not cross. *)
     val print : Format.formatter -> t -> unit
   end