minor : Adding comment

src/transform/introduction.ml

@@ -20,6 +20,9 @@ open Ty
 open Term
 open Decl
 
+(* This label is used to stop the introduction transformation to introduce
+variables past it. It is generated by software that uses why3 as a backend
+such as SPARK (for counterexamples) *)
 let stop_intro = Ident.create_label "stop_intro"
 
 let rec intros pr f =