diff --git a/C/cdmgr.c b/C/cdmgr.c
index e5fafd92f..89b245687 100644
--- a/C/cdmgr.c
+++ b/C/cdmgr.c
@@ -11,8 +11,38 @@
 * File:		cdmgr.c							 *
 * comments:	Code manager						 *
 *									 *
-* Last rev:     $Date: 2005-10-18 17:04:43 $,$Author: vsc $						 *
+* Last rev:     $Date: 2005-10-29 01:28:37 $,$Author: vsc $						 *
 * $Log: not supported by cvs2svn $
+* Revision 1.170  2005/10/18 17:04:43  vsc
+* 5.1:
+* - improvements to GC
+*    2 generations
+*    generic speedups
+* - new scheme for attvars
+*    - hProlog like interface also supported
+* - SWI compatibility layer
+*    - extra predicates
+*    - global variables
+*    - moved to Prolog module
+* - CLP(R) by Leslie De Koninck, Tom Schrijvers, Cristian Holzbaur, Bart
+* Demoen and Jan Wielemacker
+* - load_files/2
+*
+* from 5.0.1
+*
+* - WIN32 missing include files (untested)
+* - -L trouble (my thanks to Takeyuchi Shiramoto-san)!
+* - debugging of backtrable user-C preds would core dump.
+* - redeclaring a C-predicate as Prolog core dumps.
+* - badly protected  YapInterface.h.
+* - break/0 was failing at exit.
+* - YAP_cut_fail and YAP_cut_succeed were different from manual.
+* - tracing through data-bases could core dump.
+* - cut could break on very large computations.
+* - first pass at BigNum issues (reported by Roberto).
+* - debugger could get go awol after fail port.
+* - weird message on wrong debugger option.
+*
 * Revision 1.169  2005/10/15 02:05:57  vsc
 * fix for trying to add clauses to a C pred.
 *
@@ -4483,6 +4513,30 @@ p_predicate_erased_statistics(void)
 }
 #endif /* DEBUG */
 
+static int
+p_program_continuation(void)
+{
+  PredEntry *pe = EnvPreg(((CELL *)ENV[E_E])[E_CP]);
+  if (pe->ModuleOfPred) {
+    if (!Yap_unify(ARG1,pe->ModuleOfPred))
+      return FALSE;
+  } else {
+    if (!Yap_unify(ARG1,TermProlog))
+      return FALSE;
+  }
+  if (pe->ArityOfPE) {
+    if (!Yap_unify(ARG2,MkAtomTerm(NameOfFunctor(pe->FunctorOfPred))))
+      return FALSE;
+    if (!Yap_unify(ARG3,MkIntegerTerm(ArityOfFunctor(pe->FunctorOfPred))))
+      return FALSE;
+  } else {
+    if (!Yap_unify(ARG2,MkAtomTerm((Atom)pe->FunctorOfPred)))
+      return FALSE;
+    if (!Yap_unify(ARG3,MkIntTerm(0)))
+      return FALSE;
+  }
+  return TRUE;
+}
 
 void 
 Yap_InitCdMgr(void)
@@ -4539,6 +4593,7 @@ Yap_InitCdMgr(void)
   Yap_InitCPred("$continue_static_clause", 5, p_continue_static_clause, SafePredFlag|SyncPredFlag|HiddenPredFlag);
   Yap_InitCPred("$static_pred_statistics", 5, p_static_pred_statistics, SyncPredFlag|HiddenPredFlag);
   Yap_InitCPred("$p_nth_clause", 4, p_nth_clause, SyncPredFlag|HiddenPredFlag);
+  Yap_InitCPred("$program_continuation", 3, p_program_continuation, SafePredFlag|SyncPredFlag|HiddenPredFlag);
 #ifdef DEBUG
   Yap_InitCPred("predicate_erased_statistics", 5, p_predicate_erased_statistics, SyncPredFlag);
 #endif
diff --git a/C/tracer.c b/C/tracer.c
index 49c777b4e..8ced8fa31 100644
--- a/C/tracer.c
+++ b/C/tracer.c
@@ -120,12 +120,6 @@ low_level_trace(yap_low_level_port port, PredEntry *pred, CELL *args)
   /*  extern int gc_calls; */
 
   vsc_count++;
-  if (vsc_count == 21857LL) {
-    jmp_deb(1);
-  }
-  if (vsc_count < 21800LL) {
-    return;
-  }
 #ifdef COMMENTED
   //  if (vsc_count == 218280)
   //    vsc_xstop = 1;
diff --git a/changes-5.1.html b/changes-5.1.html
index 7117d0a1b..98bf8873f 100644
--- a/changes-5.1.html
+++ b/changes-5.1.html
@@ -16,6 +16,8 @@
 
 <h2>Yap-5.1.0:</h2>
 <ul>
+<li> FIXED: make undefined excetpion more ISO like. </li>
+<li> FIXED: add missing dependency in chr's Makefile.in. </li>
 <li> FIXED: fix corruption in ypp.yap. </li>
 <li> FIXED: ~@ makes format a meta-predicate. </li>
 <li> FIXED: format should do immediate exit on exception (from Prolog call). </li>
diff --git a/distribute b/distribute
index 8a35c5b9e..90fd160b7 100755
--- a/distribute
+++ b/distribute
@@ -1,7 +1,7 @@
 #/bin/bash
 # Guess what: this code works for me!
 
-version="Yap-5.0.0"
+version="Yap-5.1.0"
 splat
 cd C
 splat
@@ -21,7 +21,7 @@ cd ../clpr
 splat
 cd ../clp
 splat
-cd JPL
+cd ../JPL
 splat
 cd src
 splat
diff --git a/pl/errors.yap b/pl/errors.yap
index 2765e2874..4d1d1f092 100644
--- a/pl/errors.yap
+++ b/pl/errors.yap
@@ -11,8 +11,11 @@
 * File:		errors.yap						 *
 * comments:	error messages for YAP					 *
 *									 *
-* Last rev:     $Date: 2005-10-28 17:38:50 $,$Author: vsc $						 *
+* Last rev:     $Date: 2005-10-29 01:28:37 $,$Author: vsc $						 *
 * $Log: not supported by cvs2svn $
+* Revision 1.67  2005/10/28 17:38:50  vsc
+* sveral updates
+*
 * Revision 1.66  2005/10/18 17:04:43  vsc
 * 5.1:
 * - improvements to GC
@@ -589,9 +592,9 @@ print_message(Level, Mss) :-
 '$output_error_message'(existence_error(queue,F), W) :-
 	format(user_error,'% EXISTENCE ERROR- ~w could not open message queue ~w~n',
 	[W,F]).
-'$output_error_message'(existence_error(procedure,P), _) :-
-	format(user_error,'% EXISTENCE ERROR- procedure ~w undefined~n',
-	[P]).
+'$output_error_message'(existence_error(procedure,P), context(Call,Parent)) :-
+	format(user_error,'% EXISTENCE ERROR- procedure ~w is undefined, called from context  ~w~n%                  Goal was ~w~n',
+	[P,Parent,Call]).
 '$output_error_message'(existence_error(source_sink,F), W) :-
 	format(user_error,'% EXISTENCE ERROR- ~w could not find file ~w~n',
 	[W,F]).
diff --git a/pl/utils.yap b/pl/utils.yap
index 3b3c0f50d..1ad103e00 100644
--- a/pl/utils.yap
+++ b/pl/utils.yap
@@ -456,8 +456,10 @@ unknown(V0,V) :-
 	arg(1,X,A),
 	recorda('$unknown','$unknown'(A,M:X),_).
 
-'$unknown_error'(P) :-
-	'$do_error'(unknown,P).
+'$unknown_error'(Mod:Goal) :-
+	functor(Goal,Name,Arity),
+	'$program_continuation'(PMod,PName,PAr),
+	'$do_error'(existence_error(procedure,Name/Arity),context(Mod:Goal,PMod:PName/PAr)).
 
 '$unknown_warning'(P) :-
 	P=M:F,