From: Claudio Sacerdoti Coen <claudio.sacerdoticoen@unibo.it>
Date: Wed, 17 Dec 2003 11:27:30 +0000 (+0000)
Subject: * comments improved an possible code weakness (i.e. less than first order
X-Git-Tag: V_0_5_1_3~61
X-Git-Url: http://matita.cs.unibo.it/gitweb/?a=commitdiff_plain;h=d28e499e10fda9fca6cd7767822f018aa2d5950d;p=helm.git

* comments improved an possible code weakness (i.e. less than first order
  unification detected) made explicit in the comment and with a warning.
---

diff --git a/helm/ocaml/cic_unification/cicUnification.ml b/helm/ocaml/cic_unification/cicUnification.ml
index ad1b47206..7221dd0a8 100644
--- a/helm/ocaml/cic_unification/cicUnification.ml
+++ b/helm/ocaml/cic_unification/cicUnification.ml
@@ -31,9 +31,15 @@ exception OpenTerm;;
 
 (**** DELIFT ****)
 
-(* the delift function takes in input an ordered list of integers [n1,...,nk]
-   and a term t, and relocates rel(nk) to k. Typically, the list of integers 
-   is a parameter of a metavariable occurrence. *)
+(* the delift function takes in input an ordered list of optional terms       *)
+(* [t1,...,tn] and a term t, and substitutes every tk = Some (rel(nk)) with   *)
+(* rel(k). Typically, the list of optional terms is the explicit substitution *)(* that is applied to a metavariable occurrence and the result of the delift  *)
+(* function is a term the implicit variable can be substituted with to make   *)
+(* the term [t] unifiable with the metavariable occurrence.                   *)
+(* In general, the problem is undecidable if we consider equivalence in place *)
+(* of alpha convertibility. Our implementation, though, is even weaker than   *)
+(* alpha convertibility, since it replace the term [tk] if and only if [tk]   *)
+(* is a Rel (missing all the other cases). Does this matter in practice?      *)
 
 exception NotInTheList;;
 
@@ -143,7 +149,18 @@ let delift context metasenv l t =
         in
          C.CoFix (i, liftedfl)
   in
-    let res = deliftaux 0 t in
+   let res =
+    try
+     deliftaux 0 t
+    with
+     NotInTheList ->
+      (* This is the case where we fail even first order unification. *)
+      (* The reason is that our delift function is weaker than first  *)
+      (* order (in the sense of alpha-conversion). See comment above  *)
+      (* related to the delift function.                              *)
+prerr_endline "!!!!!!!!!!! First Order UnificationFailed, but maybe it could have been successful even in a first order setting (no conversion, only alpha convertibility)! Please, implement a better delift function !!!!!!!!!!!!!!!!" ;
+      raise UnificationFailed
+   in
     res, restrict !to_be_restricted metasenv
 ;;