diff --git a/C/index.c b/C/index.c
index e36a83f90..128443d9c 100755
--- a/C/index.c
+++ b/C/index.c
@@ -1118,6 +1118,11 @@ move_next(ClauseDef *clause, UInt regno)
   switch (op) {
   case _native_me:
     return;
+#if THREADS
+  case _unlock_lu:
+    clause->CurrentCode = NEXTOP(cl,e);
+    return;
+#endif
   case _p_db_ref_x:
   case _p_float_x:
     if (wreg == cl->y_u.xl.x) {
@@ -1386,6 +1391,11 @@ add_arg_info(ClauseDef *clause, PredEntry *ap, UInt argno)
     case _run_eam:
       cl = NEXTOP(cl,os);
       break;
+#endif   
+#ifdef THREADS
+    case _unlock_lu:
+      cl = NEXTOP(cl,e);
+      break;
 #endif   
     case _get_dbterm:
       cl = NEXTOP(cl,xc);
@@ -1508,6 +1518,9 @@ skip_to_arg(ClauseDef *clause, PredEntry *ap, UInt argno, int at_point)
     case _unify_struct_write:
       cl = NEXTOP(cl,ofa);
       break;      
+#ifdef THREADS
+    case _unlock_lu:
+#endif
     case _pop:
       cl = NEXTOP(cl,e);
       break;            
@@ -5580,9 +5593,6 @@ add_to_index(struct intermediates *cint, int first, path_stack_entry *sp, Clause
     case _lock_lu:
       ipc = NEXTOP(ipc,p);
       break;
-    case _unlock_lu:
-      ipc = NEXTOP(ipc,e);
-      break;
     case _op_fail:
       while ((--sp)->flag != block_entry);
       *sp->uip.cle.entry_code = cls->Code;
diff --git a/H/findclause.h b/H/findclause.h
index 569ec7c68..ba97357fa 100644
--- a/H/findclause.h
+++ b/H/findclause.h
@@ -625,6 +625,9 @@
     case _lock_lu:
       cl = NEXTOP(cl,p);
       break;
+    case _unlock_lu:
+      cl = NEXTOP(cl,e);
+      break;
     case _call_bfunc_xx:
       cl = NEXTOP(cl,plxxs);
       break;
diff --git a/H/headclause.h b/H/headclause.h
index 31d64ab96..7d8b68153 100644
--- a/H/headclause.h
+++ b/H/headclause.h
@@ -7,8 +7,11 @@
     op_numbers op = Yap_op_from_opcode(cl->opc);
     switch (op) {
     case _ensure_space:
-      cl = NEXTOP(cl,Osbpa);
-      break;
+       cl = NEXTOP(cl,Osbpa);
+       break;
+    case _unlock_lu:
+       cl = NEXTOP(cl,e);
+       break;
     case _native_me:
       cl = NEXTOP(cl,aFlp);
       break;
diff --git a/misc/buildops b/misc/buildops
index 33003c335..005f21497 100644
--- a/misc/buildops
+++ b/misc/buildops
@@ -171,6 +171,8 @@ special_op("table_answer_resolution_completion","THREADS_CONSUMER_SHARING").
 special_op("run_eam","BEAM").
 special_op("retry_eam","BEAM").
 special_op("thread_local","THREADS").
+special_op("lock_lu","THREADS").
+special_op("unlock_lu","THREADS").
 
 output_rclause(C) :-
 	setof(T,O^op(T,O),Types),
@@ -196,7 +198,7 @@ output_typeinfo(_,_).
 
 % tables require access to the table info. 
 special_formats(C,"e") :- !,
-	format(C,'      if (op == _Nstop || op == _copy_idb_term || op == _unify_idb_term) return;~n',[]).
+	format(C,'      if (pp == _nstop || op == _copy_idb_term || op == _unify_idb_term) return;~n',[]).
 special_formats(C,"l") :- !,
 	format(C,'      if (op == _Ystop) return;~n',[]).
 special_formats(C,"sssl") :- !,
@@ -858,6 +860,8 @@ opinfo("p_nonvar_y",[body,ifthenelse]).
 opinfo("save_b_x",[body,new("x")]).
 opinfo("save_b_y",[body,new("y")]).
 opinfo("ensure_space",[]).
+opinfo("lock_lu",[body]).
+opinfo("unlock_lu",[]).
 opinfo("write_x_loc",[body]).
 opinfo("write_x_var",[body,new("x")]).
 opinfo("write_y_var",[body,new("y")]).
@@ -1128,7 +1132,6 @@ opinfo("put_i_val_x",[body]).
 opinfo("put_fi_val_y",[body]).
 opinfo("put_f_val_y",[body]).
 opinfo("put_i_val_y",[body]).
-opinfo("lock_lu",[body]).
 opinfo("call_bfunc_xx",[body]).
 opinfo("call_bfunc_yx",[body]).
 opinfo("call_bfunc_xy",[body]).
@@ -1148,12 +1151,14 @@ output_save_clause(S) :-
 output_save_clause(_).
 
 output_save_type(S, T) :-
+(T=="e" -> start_low_level_trace; true),
 	format(S,'      /* instructions type ~s */~n',[T]),
 	setof(Op,op(T,Op),Ops),
 	dump_ops(S,Ops),
 	% then the code for every instruction with this header.
 	tinfo(T, Desc),
 	output_save(S, T, Desc, T),
+(T=="e" -> stop_low_level_trace; true),
 	format(S,'      pc = NEXTOP(pc,~s);~n',[T]),
 	format(S,'      break;~n',[]).