fix nth_instance: fix bugs and actually add two versions (obs from Theofrastos Mantadelis)

C/dbase.c

@@ -3993,7 +3993,7 @@ find_next_clause(DBRef ref0)
       UNLOCK(cl->ClLock);
     }
 #else 
-    if (!DynamicFlags(newp) & InUseMask) {
+    if (!(DynamicFlags(newp) & InUseMask)) {
       DynamicFlags(newp) |= InUseMask;
       TRAIL_CLREF(ClauseCodeToDynamicClause(newp));
     }
@@ -5489,7 +5489,7 @@ Yap_InitDBPreds(void)
   Yap_InitCPred("total_erased", 4, p_total_erased, SyncPredFlag);
   Yap_InitCPred("key_erased_statistics", 5, p_key_erased_statistics, SyncPredFlag);
   Yap_InitCPred("heap_space_info", 3, p_heap_space_info, SyncPredFlag);
-  Yap_InitCPred("$nth_instance", 4, p_nth_instance, SyncPredFlag);
+  Yap_InitCPred("$nth_instance", 3, p_nth_instance, SyncPredFlag);
   Yap_InitCPred("$nth_instancep", 3, p_nth_instancep, SyncPredFlag|HiddenPredFlag);
   Yap_InitCPred("$jump_to_next_dynamic_clause", 0, p_jump_to_next_dynamic_clause, SyncPredFlag|HiddenPredFlag);
   Yap_InitCPred("$install_thread_local", 2, p_install_thread_local, SafePredFlag|HiddenPredFlag);

C/index.c

@@ -7132,12 +7132,12 @@ Yap_NthClause(PredEntry *ap, Int ncls)
   yamop **jlbl = NULL;
 
   /* search every clause */
-  if (ncls == 1)
+  if (ncls > ap->cs.p_code.NOfClauses)
+    return NULL;
+  else if (ncls == 1)
     return to_clause(ap->cs.p_code.FirstClause,ap);
   else if (ncls == ap->cs.p_code.NOfClauses)
     return to_clause(ap->cs.p_code.LastClause,ap);
-  else if (ncls > ap->cs.p_code.NOfClauses)
-    return NULL;
   else if (ncls < 0)
     return NULL;
   

docs/yap.tex

@@ -5966,16 +5966,6 @@ elements of the internal data-base that match the key.
 match the reference.
 @end itemize
 
-@item nth_instance(?@var{K},?@var{Index},@var{T},?@var{R})
-@findex nth_recorded/3
-@saindex nth_recorded/3
-@cnindex nth_recorded/3
-Fetches the @var{Index}nth entry in the internal database under the
-key @var{K}. Entries are numbered from one. If the key @var{K} are the
-@var{Index} are bound, a reference is unified with @var{R}. Otherwise,
-the reference @var{R} must be given, and the term the system will find
-the matching key and index. 
-
 @item erase(+@var{R})
 @findex erase/1
 @saindex erase/1
@@ -6015,6 +6005,27 @@ Defines the relation: @var{K} is a currently defined database key whose
 name is the atom @var{A}. It can be used to generate all the keys for
 the internal data-base.
 
+@item nth_instance(?@var{Key},?@var{Index},?@var{R})
+@findex nth_instance/3
+@saindex nth_instance/3
+@cnindex nth_instance/3
+Fetches the @var{Index}nth entry in the internal database under the key
+@var{Key}. Entries are numbered from one. If the key @var{Key} or the
+@var{Index} are bound, a reference is unified with @var{R}. Otherwise,
+the reference @var{R} must be given, and YAP will find
+the matching key and index.
+
+
+@item nth_instance(?@var{Key},?@var{Index},@var{T},?@var{R})
+@findex nth_instance/4
+@saindex nth_instance/4
+@cnindex nth_instance/4
+Fetches the @var{Index}nth entry in the internal database under the key
+@var{Key}. Entries are numbered from one. If the key @var{Key} or the
+@var{Index} are bound, a reference is unified with @var{R}. Otherwise,
+the reference @var{R} must be given, and YAP will find
+the matching key and index.
+
 @item key_statistics(+@var{K},-@var{Entries},-@var{Size},-@var{IndexSize})
 @findex key_statistics/4
 @snindex key_statistics/4

pl/utils.yap

@@ -472,12 +472,20 @@ callable(V) :- var(V), !, fail.
 callable(V) :- atom(V), !.
 callable(V) :- functor(V,_,Ar), Ar > 0.
 
-nth_instance(X,Y,Z) :-
+nth_instance(Key,Index,Ref) :-
-	nonvar(X), var(Y), var(Z), !,
+	nonvar(Key), var(Index), var(Ref), !,
-	recorded(X,_,Z),
+	recorded(Key,_,Ref),
-	'$nth_instance'(_,Y,Z).
+	'$nth_instance'(_,Index,Ref).
-nth_instance(X,Y,Z) :-
+nth_instance(Key,Index,Ref) :-
-	'$nth_instance'(X,Y,Z).
+	'$nth_instance'(Key,Index,Ref).
+
+nth_instance(Key,Index,T,Ref) :-
+	nonvar(Key), var(Index), var(Ref), !,
+	recorded(Key,T,Ref),
+	'$nth_instance'(_,Index,Ref).
+nth_instance(Key,Index,T,Ref) :-
+	'$nth_instance'(Key,Index,Ref),
+	instance(Ref,T).
 
 nb_current(GlobalVariable, Val) :-
 	var(GlobalVariable), !,