diff --git a/C/absmi.c b/C/absmi.c
index 3ce1159e7..5dcb178b0 100755
--- a/C/absmi.c
+++ b/C/absmi.c
@@ -7530,10 +7530,7 @@ Yap_absmi(int inp)
       Yap_PrologMode = UserCCallMode;
       {
 	PredEntry *p = PREG->u.Osbpp.p;
-#ifdef LOW_LEVEL_TRACER
-	if (Yap_do_low_level_trace)
-	  low_level_trace(enter_pred,p,XREGS+1);
-#endif	/* LOW_LEVEL_TRACE */
+
 	PREG = NEXTOP(PREG, Osbpp);
 	saveregs();
 	save_machine_regs();
@@ -7541,6 +7538,7 @@ Yap_absmi(int inp)
 	SREG = (CELL *) YAP_Execute(p, p->cs.f_code);
 	EX = 0L;
       }
+      Yap_CloseSlots();
 
       restore_machine_regs();
       setregs();
@@ -7714,6 +7712,7 @@ Yap_absmi(int inp)
       restore_machine_regs();
       setregs();
       Yap_PrologMode = UserMode;
+      Yap_CloseSlots();
       if (!SREG) {
 	FAIL();
       }
@@ -7756,6 +7755,7 @@ Yap_absmi(int inp)
       restore_machine_regs();
       setregs();
       Yap_PrologMode = UserMode;
+      Yap_CloseSlots();
       if (!SREG) {
 #ifdef CUT_C
 	/* Removes the cut functions from the stack
diff --git a/C/adtdefs.c b/C/adtdefs.c
index 66553230e..d27947c68 100755
--- a/C/adtdefs.c
+++ b/C/adtdefs.c
@@ -1382,6 +1382,7 @@ Yap_NewSlots(int n)
     n--;
   }
   ASP[0] = MkIntTerm(old_slots+oldn);
+  CurSlot = LCL0-ASP;
   return((ASP+1)-LCL0);
 }
 
@@ -1391,6 +1392,7 @@ Yap_InitSlot(Term t)
   Int old_slots = IntOfTerm(ASP[0]);
   *ASP = t;
   ASP--;
+  CurSlot ++;
   ASP[0] = MkIntTerm(old_slots+1);
   return((ASP+1)-LCL0);
 }
@@ -1403,6 +1405,7 @@ Yap_RecoverSlots(int n)
     return FALSE;
   }
   ASP += n;
+  CurSlot -= n;
   ASP[0] = MkIntTerm(old_slots-n);
   return TRUE;
 }
diff --git a/C/c_interface.c b/C/c_interface.c
index e023bd9c1..985e3efb1 100755
--- a/C/c_interface.c
+++ b/C/c_interface.c
@@ -1955,6 +1955,7 @@ YAP_RestartGoal(void)
     Yap_PrologMode = UserCCallMode;
     if (out == FALSE) {
       /* cleanup */
+      Yap_CloseSlots();
       Yap_trust_last();
       Yap_AllowRestart = FALSE;
     }
@@ -2719,6 +2720,7 @@ YAP_Predicate(Atom a, UInt arity, Term m)
   } else {
     Functor f = Yap_MkFunctor(a, arity);
     return((void *)RepPredProp(PredPropByFunc(f,m)));
+    fprintf(stderr,"here\n");
   }
 } 
 
diff --git a/C/cdmgr.c b/C/cdmgr.c
index bb4bb7a52..c95cb3124 100644
--- a/C/cdmgr.c
+++ b/C/cdmgr.c
@@ -3299,7 +3299,10 @@ search_for_static_predicate_in_use(PredEntry *p, int check_everything)
 	 only for retracts
       */
       while (env_ptr && b_ptr > (choiceptr)env_ptr) {
-	PredEntry *pe = EnvPreg((yamop *)env_ptr[E_CP]);
+	yamop *cp = (yamop *)env_ptr[E_CP];
+	PredEntry *pe;
+
+	pe = EnvPreg(cp);
 	if (p == pe) return(TRUE);
 	if (env_ptr != NULL)
 	  env_ptr = (CELL *)(env_ptr[E_E]);
diff --git a/C/exec.c b/C/exec.c
index 3a7dec27a..873e7b54d 100644
--- a/C/exec.c
+++ b/C/exec.c
@@ -994,6 +994,7 @@ exec_absmi(int top)
   } else {
     Yap_PrologMode = UserMode;
   }
+  Yap_CloseSlots();
   out = Yap_absmi(0);
   Yap_StartSlots();
   return out;
@@ -1049,6 +1050,7 @@ init_stack(int arity, CELL *pt, int top, choiceptr saved_b)
 #endif
   YENV[E_CB] = Unsigned (B);
   CP = YESCODE;
+  Yap_StartSlots();
 }
 
 static Term
@@ -1200,7 +1202,6 @@ void
 Yap_trust_last(void)
 {
   ASP  = B->cp_env;
-  P    = (yamop *)(B->cp_env[E_CP]);
   CP   = B->cp_cp;
   H    = B->cp_h;
 #ifdef DEPTH_LIMIT
@@ -1209,6 +1210,7 @@ Yap_trust_last(void)
   YENV= ASP = B->cp_env;
   ENV  = (CELL *)((B->cp_env)[E_E]);
   B    = B->cp_b;
+  P    = (yamop *)(ENV[E_CP]);
   if (B) {
     SET_BB(B);
     HB = PROTECT_FROZEN_H(B);
@@ -1621,11 +1623,9 @@ Yap_InitYaamRegs(void)
   CreepFlag = CalculateStackGap();
   UNLOCK(SignalLock);
   EX = 0L;
-  /* for slots to work */
-  Yap_StartSlots();
   init_stack(0, NULL, TRUE, NULL);
   /* the first real choice-point will also have AP=FAIL */ 
-  Yap_StartSlots();
+  CurSlot = 0;
   GlobalArena = TermNil;
   h0var = MkVarTerm();
 #if COROUTINING
diff --git a/C/heapgc.c b/C/heapgc.c
index 74f4374e5..fd7f6fafe 100755
--- a/C/heapgc.c
+++ b/C/heapgc.c
@@ -1837,14 +1837,22 @@ mark_trail(tr_fr_ptr trail_ptr, tr_fr_ptr trail_base, CELL *gc_H, choiceptr gc_B
 
 
 static void
-mark_slots(CELL *ptr)
+mark_slots(void)
 {
-  Int ns = IntOfTerm(*ptr);
-  ptr++;
-  while (ns > 0) {
-    mark_external_reference(ptr);
+  Int curslot = CurSlot;
+  while (curslot) {
+    CELL *ptr = LCL0-curslot;
+    Int ns = IntegerOfTerm(*ptr);
+
     ptr++;
-    ns--;
+    while (ns > 0) {
+      //      Yap_DebugPlWrite(ptr);
+      //fprintf(stderr,"\n");
+      mark_external_reference(ptr);
+      ptr++;
+      ns--;
+    }
+    curslot = IntegerOfTerm(*ptr);
   }
 }
 
@@ -1978,7 +1986,6 @@ mark_choicepoints(register choiceptr gc_B, tr_fr_ptr saved_TR, int very_verbose)
     restart_cp:
       switch (opnum) {
       case _Nstop:
-	mark_slots(gc_B->cp_env);
 	if (gc_B->cp_b != NULL) {
 	  nargs = 0;
 	  break;
@@ -2777,20 +2784,25 @@ sweep_environments(CELL_PTR gc_ENV, OPREG size, CELL *pvbmap)
 }
 
 static void
-sweep_slots(CELL *ptr)
+sweep_slots(void)
 {
-  Int ns = IntOfTerm(*ptr);
-  ptr++;
-  while (ns > 0) {
-    CELL cp_cell = *ptr;
-    if (MARKED_PTR(ptr)) {
-      UNMARK(ptr);
-      if (HEAP_PTR(cp_cell)) {
-	into_relocation_chain(ptr, GET_NEXT(cp_cell));
-      }
-    }
+  Int curslot = CurSlot;
+  while (curslot) {
+    CELL *ptr = LCL0-curslot;
+    Int ns = IntOfTerm(*ptr);
     ptr++;
-    ns--;
+    while (ns > 0) {
+      CELL cp_cell = *ptr;
+      if (MARKED_PTR(ptr)) {
+	UNMARK(ptr);
+	if (HEAP_PTR(cp_cell)) {
+	  into_relocation_chain(ptr, GET_NEXT(cp_cell));
+	}
+      }
+      ptr++;
+      ns--;
+    }
+    curslot = IntegerOfTerm(*ptr);
   }
 }
 
@@ -2870,7 +2882,6 @@ sweep_choicepoints(choiceptr gc_B)
       sweep_environments(gc_B->cp_env,
 			 EnvSizeInCells,
 			 NULL);
-      sweep_slots(gc_B->cp_env);
       if (gc_B->cp_b != NULL) {
 	break;
       } else
@@ -3606,8 +3617,7 @@ marking_phase(tr_fr_ptr old_TR, CELL *current_env, yamop *curp)
   cont_top = (cont *)db_vec;
   /* These two must be marked first so that our trail optimisation won't lose
      values */
-  if (Yap_PrologMode & UserCCallMode)
-    mark_slots(ASP);
+  mark_slots();
   mark_regs(old_TR);		/* active registers & trail */
   /* active environments */
   mark_environments(current_env, EnvSize(curp), EnvBMap(curp));
@@ -3660,8 +3670,7 @@ compaction_phase(tr_fr_ptr old_TR, CELL *current_env, yamop *curp)
       sweep_oldgen(HGEN, CurrentH0);
     }
   }
-  if (Yap_PrologMode & UserCCallMode)
-    sweep_slots(ASP);
+  sweep_slots();
   sweep_environments(current_env, EnvSize(curp), EnvBMap(curp));
   sweep_choicepoints(B);
   sweep_trail(B, old_TR);
diff --git a/C/iopreds.c b/C/iopreds.c
index 659169bef..490f7e63a 100755
--- a/C/iopreds.c
+++ b/C/iopreds.c
@@ -3660,6 +3660,7 @@ int beam_write (void)
 {
   Yap_StartSlots();
   Yap_plwrite (ARG1, Stream[Yap_c_output_stream].stream_wputc, 0, 1200);
+  Yap_CloseSlots();
   if (EX != 0L) {
     Term ball = EX;
     EX = 0L;
@@ -3679,6 +3680,7 @@ p_write (void)
      we cannot make recursive Prolog calls */
   Yap_StartSlots();
   Yap_plwrite (ARG2, Stream[Yap_c_output_stream].stream_wputc, flags, 1200);
+  Yap_CloseSlots();
   if (EX != 0L) {
     Term ball = EX;
     EX = 0L;
@@ -3697,6 +3699,7 @@ p_write_prio (void)
      we cannot make recursive Prolog calls */
   Yap_StartSlots();
   Yap_plwrite (ARG3, Stream[Yap_c_output_stream].stream_wputc, flags, (int)IntOfTerm(Deref(ARG2)));
+  Yap_CloseSlots();
   if (EX != 0L) {
     Term ball = EX;
     EX = 0L;
@@ -3720,6 +3723,7 @@ p_write2_prio (void)
      we cannot make recursive Prolog calls */
   Yap_StartSlots();
   Yap_plwrite (ARG4, Stream[Yap_c_output_stream].stream_wputc, (int) IntOfTerm (Deref (ARG2)), (int) IntOfTerm (Deref (ARG3)));
+  Yap_CloseSlots();
   Yap_c_output_stream = old_output_stream;
   if (EX != 0L) {
     Term ball = EX;
@@ -3744,6 +3748,7 @@ p_write2 (void)
      we cannot make recursive Prolog calls */
   Yap_StartSlots();
   Yap_plwrite (ARG3, Stream[Yap_c_output_stream].stream_wputc, (int) IntOfTerm (Deref (ARG2)), 1200);
+  Yap_CloseSlots();
   Yap_c_output_stream = old_output_stream;
   if (EX != 0L) {
     Term ball = EX;
@@ -5089,6 +5094,7 @@ format(volatile Term otail, volatile Term oargs, int sno)
 	  goto do_type_atom_error;
 	Yap_StartSlots();
 	Yap_plwrite (t, f_putc, Handle_vars_f|To_heap_f, 1200);
+	Yap_CloseSlots();
 	FormatInfo = &finfo;
 	break;
       case 'c':
@@ -5338,8 +5344,8 @@ format(volatile Term otail, volatile Term oargs, int sno)
 	    t = targs[targ++];
 	    Yap_StartSlots();
 	    Yap_plwrite (t, f_putc, Quote_illegal_f|Ignore_ops_f|To_heap_f , 1200);
+	    Yap_CloseSlots();
 	    FormatInfo = &finfo;
-	    ASP++;
 	    break;
 	  case '@':
 	    t = targs[targ++];
@@ -5368,6 +5374,7 @@ format(volatile Term otail, volatile Term oargs, int sno)
 		goto do_default_error;
 	      }
 	    }
+	    Yap_CloseSlots();
 	    break;
 	  case 'p':
 	    if (targ > tnum-1 || has_repeats)
@@ -5381,6 +5388,7 @@ format(volatile Term otail, volatile Term oargs, int sno)
 	      args = Yap_GetFromSlot(sl);
 	      Yap_RecoverSlots(1);
 	    }
+	    Yap_CloseSlots();
 	    if (EX != 0L) {
 	      Term ball;
 
@@ -5399,7 +5407,6 @@ format(volatile Term otail, volatile Term oargs, int sno)
 	      Yap_JumpToEnv(ball);
 	      return FALSE;
 	    }
-	    ASP++;
 	    break;
 	  case 'q':
 	    if (targ > tnum-1 || has_repeats)
@@ -5407,8 +5414,8 @@ format(volatile Term otail, volatile Term oargs, int sno)
 	    t = targs[targ++];
 	    Yap_StartSlots();
 	    Yap_plwrite (t, f_putc, Handle_vars_f|Quote_illegal_f|To_heap_f, 1200);
+	    Yap_CloseSlots();
 	    FormatInfo = &finfo;
-	    ASP++;
 	    break;
 	  case 'w':
 	    if (targ > tnum-1 || has_repeats)
@@ -5416,8 +5423,8 @@ format(volatile Term otail, volatile Term oargs, int sno)
 	    t = targs[targ++];
 	    Yap_StartSlots();
 	    Yap_plwrite (t, f_putc, Handle_vars_f|To_heap_f, 1200);
+	    Yap_CloseSlots();
 	    FormatInfo = &finfo;
-	    ASP++;
 	    break;
 	  case '~':
 	    if (has_repeats)
@@ -6262,10 +6269,10 @@ Yap_TermToString(Term t, char *s, unsigned int sz, int flags)
 
   if (sno < 0)
     return FALSE;
-  Yap_StartSlots();
   Yap_c_output_stream = sno;
   Yap_StartSlots();
   Yap_plwrite (t, Stream[sno].stream_wputc, flags, 1200);
+  Yap_CloseSlots();
   s[Stream[sno].u.mem_string.pos] = '\0';
   Stream[sno].status = Free_Stream_f;
   Yap_c_output_stream = old_output_stream;
diff --git a/H/Regs.h b/H/Regs.h
index adf6ff594..6b015d6af 100644
--- a/H/Regs.h
+++ b/H/Regs.h
@@ -80,6 +80,7 @@ EXTERN void save_B(void);
 
 typedef struct
   {
+    Int     CurSlot_;
     CELL    CreepFlag_;		/* 13                                         */
     CELL   *HB_;		/* 4 heap (global) stack top at latest c.p.   */
 #if defined(SBA) || defined(TABLING)
@@ -609,11 +610,10 @@ EXTERN inline void restore_B(void) {
 
 #endif
 
+#define	CurSlot       Yap_REGS.CurSlot_
 #define	AuxBase       Yap_REGS.AuxBase_
 #define	AuxSp         Yap_REGS.AuxSp_
 #define	AuxTop        Yap_REGS.AuxTop_
-#define TopB          Yap_REGS.TopB_
-#define DelayedB      Yap_REGS.DelayedB_
 #define EX            Yap_REGS.EX_
 #define DEPTH	      Yap_REGS.DEPTH_
 #if defined(SBA) || defined(TABLING)
diff --git a/H/Yap.h b/H/Yap.h
index f1a76f831..11997c64b 100755
--- a/H/Yap.h
+++ b/H/Yap.h
@@ -1305,4 +1305,27 @@ typedef enum
     COMPILE_ALL          /* compile all predicates */
   } yap_exec_mode;
 
+/********* slots ***********************/
+
+
+static inline void
+Yap_StartSlots(void) {
+  *--ASP = MkIntegerTerm(CurSlot);
+  *--ASP = MkIntTerm(0);
+  CurSlot = LCL0-ASP;
+}
+
+static inline void
+Yap_CloseSlots(void) {
+  Int old_slots;
+  old_slots = IntOfTerm(ASP[0]);
+  ASP += (old_slots+1);
+  CurSlot = IntegerOfTerm(*ASP);
+  ASP++;
+}
+
+static inline Int
+Yap_CurrentSlot(void) {
+  return IntOfTerm(ASP[0]);
+}
 
diff --git a/H/Yapproto.h b/H/Yapproto.h
index 576a17351..ed46bce5f 100755
--- a/H/Yapproto.h
+++ b/H/Yapproto.h
@@ -59,8 +59,6 @@ Term	STD_PROTO(Yap_NWideStringToDiffListOfAtoms,(wchar_t *, Term, size_t));
 int     STD_PROTO(Yap_AtomIncreaseHold,(Atom));
 int     STD_PROTO(Yap_AtomDecreaseHold,(Atom));
 
-#define Yap_StartSlots() (*--ASP = MkIntTerm(0))
-#define Yap_CurrentSlot() IntOfTerm(ASP[0])
 Int	STD_PROTO(Yap_InitSlot,(Term));
 Int     STD_PROTO(Yap_NewSlots,(int));
 Term    STD_PROTO(Yap_GetFromSlot,(Int));
diff --git a/library/yap2swi/yap2swi.c b/library/yap2swi/yap2swi.c
index ba5551498..c26a51cda 100755
--- a/library/yap2swi/yap2swi.c
+++ b/library/yap2swi/yap2swi.c
@@ -2207,28 +2207,6 @@ X_API int PL_action(int action,...)
   return 0;
 }
 
-X_API fid_t
-PL_open_foreign_frame(void)
-{
-  return (fid_t)ASP;
-}
-
-X_API void
-PL_close_foreign_frame(fid_t f)
-{
-}
-
-X_API void
-PL_rewind_foreign_frame(fid_t f)
-{
-}
-
-X_API void
-PL_discard_foreign_frame(fid_t f)
-{
-  /* Missing: undo Trail!! */
-}
-
 X_API term_t
 PL_exception(qid_t q)
 {
@@ -2371,9 +2349,52 @@ typedef struct open_query_struct {
   int open;
   int state;
   YAP_Term g;
+  yamop *p, *cp;
+  Int slots;
+  struct open_query_struct *old;
 } open_query;
 
-open_query execution;
+static open_query *execution = NULL;
+
+X_API fid_t
+PL_open_foreign_frame(void)
+{
+  open_query *new = (open_query *)malloc(sizeof(open_query));
+  if (!new) return 0;
+  new->old = execution;
+  new->g = TermNil;
+  new->open = FALSE;
+  new->cp = CP;
+  new->p = P;
+  new->slots = CurSlot;
+  Yap_StartSlots();
+  execution = new;
+  return (fid_t)new;
+}
+
+X_API void
+PL_close_foreign_frame(fid_t f)
+{
+  CP = execution->cp;
+  P = execution->p;
+  CurSlot = execution->slots;
+  execution = execution->old;
+}
+
+X_API void
+PL_rewind_foreign_frame(fid_t f)
+{
+  CurSlot = execution->slots;
+}
+
+X_API void
+PL_discard_foreign_frame(fid_t f)
+{
+  CP = execution->cp;
+  P = execution->p;
+  CurSlot = execution->slots;
+  execution = execution->old;
+}
 
 X_API qid_t PL_open_query(module_t ctx, int flags, predicate_t p, term_t t0)
 {
@@ -2382,11 +2403,14 @@ X_API qid_t PL_open_query(module_t ctx, int flags, predicate_t p, term_t t0)
   Term t[2], m;
 
   /* ignore flags  and module for now */
-  if (execution.open != 0) {
+  if (execution == NULL)
+    PL_open_foreign_frame();
+  if (execution->open != 0) {
     YAP_Error(0, 0L, "only one query at a time allowed\n");
+    return FALSE;
   }
-  execution.open=1;
-  execution.state=0;
+  execution->open=1;
+  execution->state=0;
   PredicateInfo((PredEntry *)p, &yname, &arity, &m);
   t[0] = SWIModuleToModule(ctx);
   if (arity == 0) {
@@ -2395,8 +2419,8 @@ X_API qid_t PL_open_query(module_t ctx, int flags, predicate_t p, term_t t0)
     Functor f = Yap_MkFunctor(yname, arity);
     t[1] = Yap_MkApplTerm(f,arity,Yap_AddressFromSlot(t0));
   }
-  execution.g = Yap_MkApplTerm(FunctorModule,2,t);
-  return &execution;
+  execution->g = Yap_MkApplTerm(FunctorModule,2,t);
+  return execution;
 }
 
 X_API int PL_next_solution(qid_t qi)
@@ -2405,7 +2429,6 @@ X_API int PL_next_solution(qid_t qi)
 
   if (qi->open != 1) return 0;
   if (qi->state == 0) {
-
     result = YAP_RunGoal(qi->g);
   } else {
     result = YAP_RestartGoal();
diff --git a/packages/ProbLog/Makefile.in b/packages/ProbLog/Makefile.in
index 220b44d02..282d762b4 100644
--- a/packages/ProbLog/Makefile.in
+++ b/packages/ProbLog/Makefile.in
@@ -9,7 +9,8 @@ BINDIR = $(ROOTDIR)/bin
 #
 # where YAP should look for binary libraries
 #
-LIBDIR=@libdir@/Yap
+LIBDIR=@libdir@
+YAPLIBDIR=@libdir@/Yap
 #
 # where YAP should look for architecture-independent Prolog libraries
 #
diff --git a/packages/ProbLog/simplecudd/Makefile.in b/packages/ProbLog/simplecudd/Makefile.in
index 17be51be2..546e70b48 100644
--- a/packages/ProbLog/simplecudd/Makefile.in
+++ b/packages/ProbLog/simplecudd/Makefile.in
@@ -14,7 +14,8 @@ BINDIR = $(EROOTDIR)/bin
 #
 # where YAP should look for libraries
 #
-LIBDIR=@libdir@/Yap
+LIBDIR=@libdir@
+YAPLIBDIR=@libdir@/Yap
 #
 #
 CC=@CC@
@@ -60,4 +61,4 @@ clean:
 	rm -f *.o ProblogBDD Example
 
 install: default
-	$(INSTALL_PROGRAM) ProblogBDD $(DESTDIR)$(LIBDIR)
+	$(INSTALL_PROGRAM) ProblogBDD $(DESTDIR)$(YAPLIBDIR)
diff --git a/packages/jpl b/packages/jpl
index 9efaf4ce7..c6b86a4c7 160000
--- a/packages/jpl
+++ b/packages/jpl
@@ -1 +1 @@
-Subproject commit 9efaf4ce7063fbdae534b4555a80fa1373bb7e9a
+Subproject commit c6b86a4c77da30c8e3a9eafcad76a54004a779de
diff --git a/packages/plunit/Makefile.in b/packages/plunit/Makefile.in
index 185362b15..a030e3b63 100644
--- a/packages/plunit/Makefile.in
+++ b/packages/plunit/Makefile.in
@@ -19,8 +19,8 @@ srcdir=@srcdir@
 
 BINDIR = $(EROOTDIR)/bin
 LIBDIR=@libdir@
-SHAREDIR=$(EROOTDIR)/share/Yap
 YAPLIBDIR=@libdir@/Yap
+SHAREDIR=$(EROOTDIR)/share/Yap
 
 PL=@EXTEND_DYNLOADER_PATH@ $(DESTDIR)$(BINDIR)/yap $(DESTDIR)$(YAPLIBDIR)/startup.yss
 LN_S=@LN_S@