Migrate to the new type substitution API (2/3).

types/boolVar.ml

@@ -219,8 +219,10 @@ module Make (T : E) : S with type s = T.t = struct
 
   let compute ~empty ~full ~cup ~cap ~diff ~atom b =
     let rec aux = function
-      | `True -> full
+      | `True  -> full
       | `False -> empty
+      | `Split (_,`Atm x,`True,_,_) when T.equal x T.empty -> empty
+      | `Split (_,`Atm x,`True,_,_) when T.equal x T.full -> full
       | `Split(_,x, p,i,n) ->
         let x1 = atom x in
 	let p = cap x1 (aux p) in