diff --git a/pl/arith.yap b/pl/arith.yap
index ab4a3f264..fcf94b4d6 100644
--- a/pl/arith.yap
+++ b/pl/arith.yap
@@ -55,8 +55,8 @@
      + specialise versions for some built-ins, if we are aware of the
       run-time execution mode
 
-  The user has some control over this process, through some
-  built-ins and through execution flsgs.
+  The user has  control over this process, through
+  built-ins and through prolog flags.
 
 */
 
@@ -93,7 +93,7 @@ compile_expressions :- set_value('$c_arith',true).
 
 After a call to this predicate, arithmetical expressions will not be compiled.
 
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+~~~~~~~
 ?- source, do_not_compile_expressions.
 yes
 ?- [user].
@@ -111,8 +111,8 @@ p(A):-
 
 q(A):-
       A is 22.
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 
+~~~~~~~
 */
 do_not_compile_expressions :- set_value('$c_arith',[]).
 
@@ -132,6 +132,7 @@ do_c_built_in('$do_error'( Error, Goal), M, Head,
 	       throw(error(Error,M:(Head :- Goal)))
 	     ) :- !.
 do_c_built_in(system_error( Error, Goal), M, Head, ErrorG) :-
+        !,
        do_c_built_in('$do_error'( Error, Goal), M, Head, ErrorG).
 do_c_built_in(X is Y, M, H,  P) :-
         primitive(X), !,
@@ -149,8 +150,10 @@ do_c_built_in(X is Y, _, _, P) :-
 		'$drop_is'(X0, X, P0, P)
 	).
 do_c_built_in(phrase(NT,Xs),  Mod, H, NTXsNil) :-
+    !,
 	'$_arith':do_c_built_in(phrase(NT,Xs,[]), Mod, H, NTXsNil).
 do_c_built_in(phrase(NT,Xs0,Xs), Mod, _,  NewGoal) :-
+    !,
     '$c_built_in_phrase'(NT, Xs0, Xs, Mod, NewGoal ).
 
 do_c_built_in(Comp0, _, _, R) :-		% now, do it for comparisons
diff --git a/pl/boot.yap b/pl/boot.yap
index 794583e36..127d7fc9b 100644
--- a/pl/boot.yap
+++ b/pl/boot.yap
@@ -303,8 +303,8 @@ initialize_prolog :-
      'qly.yap',
      'spy.yap',
      'udi.yap'].
-%:- stop_low_level_trace.
 
+%:- start_low_level_trace.
 
 :- meta_predicate(log_event(+,:)).
 
@@ -316,7 +316,7 @@ initialize_prolog :-
 
 :-	 ['protect.yap'].
 
-version(yap,[6,3]).
+version(yap,[6,4]).
 
 :- op(1150,fx,(mode)).
 
@@ -396,6 +396,7 @@ yap_hacks:cut_by(CP) :- '$$cut_by'(CP).
 
 :- style_check([+discontiguous,+multiple,+single_var]).
 
+
 %
 % moved this to init_gc in gc.c to separate the alpha
 %
@@ -476,6 +477,8 @@ If this hook preodicate succeeds it must instantiate the  _Action_ argument to t
 
 :- dynamic user:exception/3.
 
+:- start_low_level_trace.
+
 :- ensure_loaded('../pl/pathconf.yap').
 
 :- yap_flag(user:unknown,error).
diff --git a/pl/os.yap b/pl/os.yap
index 60b7fd5ed..c8fde94fc 100644
--- a/pl/os.yap
+++ b/pl/os.yap
@@ -74,17 +74,9 @@ Prints a list of all files in the current directory.
 */
 ls :-
 	getcwd(X),
-	'$load_system_ls'(X,L),
+	list_directory(X, L),
 	'$do_print_files'(L).
 
-'$load_system_ls'(X,L) :-
-	'$undefined'(directory_files(X, L), system),
-	load_files(library(system),[silent(true)]),
-	fail.
-'$load_system_ls'(X,L) :-
-	system:directory_files(X, L).
-
-
 '$do_print_files'([]) :-
 	nl.
 '$do_print_files'([F| Fs]) :-