diff --git a/C/exec.c b/C/exec.c
index cb6b4a726..4e2e37b85 100644
--- a/C/exec.c
+++ b/C/exec.c
@@ -42,7 +42,7 @@ cp_as_integer(choiceptr cp)
 static choiceptr
 cp_from_integer(Term cpt)
 {
-  return (choiceptr)(LCL0-(CELL *)IntegerOfTerm(cpt));
+  return (choiceptr)(LCL0-IntegerOfTerm(cpt));
 }
 
 Term
@@ -82,7 +82,7 @@ CallPredicate(PredEntry *pen, choiceptr cut_pt, yamop *code) {
   ENV = YENV;
   YENV = ASP;
   YENV[E_CB] = (CELL) cut_pt;
-  return (TRUE);
+  return TRUE;
 }
 
 inline static Int
@@ -422,7 +422,7 @@ p_execute_clause(void)
 {				/* '$execute_clause'(Goal)	 */
   Term            t = Deref(ARG1);
   Term            mod = Deref(ARG2);
-  choiceptr       cp = cp_from_integer(Deref(ARG4));
+  choiceptr       cut_cp = cp_from_integer(Deref(ARG4));
   unsigned int    arity;
   Prop            pe;
   yamop *code;
@@ -478,7 +478,7 @@ p_execute_clause(void)
   } else {
     code = Yap_ClauseFromTerm(clt)->ClCode;
   }
-  return CallPredicate(RepPredProp(pe), cp, code);
+  return CallPredicate(RepPredProp(pe), cut_cp, code);
 }
 
 static Int
diff --git a/C/sysbits.c b/C/sysbits.c
index f92dc571d..674ce9c67 100644
--- a/C/sysbits.c
+++ b/C/sysbits.c
@@ -1783,19 +1783,19 @@ p_shell (void)
     bourne = TRUE;
   /* Yap_CloseStreams(TRUE); */
   if (bourne)
-    return (system (RepAtom(AtomOfTerm(t1))->StrOfAE) == 0);
+    return system(RepAtom(AtomOfTerm(t1))->StrOfAE) == 0;
   else {
-      int status = -1;
-      int child = fork ();
-      if (child == 0)
-	{			/* let the children go */
-	  if (!execl (shell, shell, "-c", RepAtom(AtomOfTerm(t1))->StrOfAE , NULL)) {
-	    exit(-1);
-	  }
-	  exit(TRUE);
-	}
-      {				/* put the father on wait */
-	int result = child < 0 ||
+    int status = -1;
+    int child = fork ();
+
+    if (child == 0) {			/* let the children go */
+      if (!execl (shell, shell, "-c", RepAtom(AtomOfTerm(t1))->StrOfAE , NULL)) {
+	exit(-1);
+      }
+      exit(TRUE);
+    }
+    {				/* put the father on wait */
+      int result = child < 0 ||
 /* vsc:I am not sure this is used, Stevens say wait returns an integer.
 #if NO_UNION_WAIT
 */
@@ -1815,10 +1815,10 @@ p_shell (void)
   shell = "msh -i";
   /* Yap_CloseStreams(); */
   system (shell);
-  return (TRUE);
+  return TRUE;
 #else
   Yap_Error (SYSTEM_ERROR,TermNil,"shell not available in this configuration");
-  return(FALSE);
+  return FALSE;
 #endif
 #endif /* HAVE_SYSTEM */
 #endif /* _MSC_VER */
diff --git a/changes-5.1.html b/changes-5.1.html
index 5890b3285..40970b3bd 100644
--- a/changes-5.1.html
+++ b/changes-5.1.html
@@ -16,6 +16,10 @@
 
 <h2>Yap-5.1.2:</h2>
 <ul>
+<li> FIXED: ! was not pruning right in p_execute_clause (obs Nicos
+  Angelopoulos).</li>
+<li> FIXED: system/1 and shell/1 should check return code (obs Nicos
+  Angelopoulos).</li>
 <li> FIXED: thread_peek_message should use aliases (obs Paulo Moura).</li>
 <li> FIXED: garbage collector should no rewrite cp_tr fields until
 after marking (obs Paulo Moura).</li>
diff --git a/library/system.yap b/library/system.yap
index 1aef4f6f8..fde3fc7b1 100644
--- a/library/system.yap
+++ b/library/system.yap
@@ -374,7 +374,8 @@ shell(Command) :-
 	G = shell(Command),
 	check_command(Command, G),
 	get_shell(Shell,Opt),
-	do_shell(Shell, Opt, Command, _, Error),
+	do_shell(Shell, Opt, Command, Status, Error),
+	Status = 0,
 	handle_system_error(Error, off, G).
 
 shell(Command, Status) :-
@@ -416,6 +417,7 @@ system(Command, Status) :-
 	G = system(Command, Status),
 	check_command(Command, G),
 	do_system(Command, Status, Error),
+	Status = 0,
 	handle_system_error(Error, off, G).
 
 sleep(Interval) :- var(Interval), !,
diff --git a/library/system/sys.c b/library/system/sys.c
index 69f868845..18bc8e97e 100644
--- a/library/system/sys.c
+++ b/library/system/sys.c
@@ -8,8 +8,13 @@
 *									 *
 **************************************************************************
 *									 *
-* $Id: sys.c,v 1.25 2006-01-17 14:10:42 vsc Exp $									 *
+* $Id: sys.c,v 1.26 2006-04-25 03:23:40 vsc Exp $									 *
 * mods:		$Log: not supported by cvs2svn $
+* mods:		Revision 1.25  2006/01/17 14:10:42  vsc
+* mods:		YENV may be an HW register (breaks some tabling code)
+* mods:		All YAAM instructions are now brackedted, so Op introduced an { and EndOp introduces an }. This is because Ricardo assumes that.
+* mods:		Fix attvars when COROUTING is undefined.
+* mods:		
 * mods:		Revision 1.24  2006/01/08 23:01:48  vsc
 * mods:		*** empty log message ***
 * mods:		
@@ -52,9 +57,7 @@
 
 #include "config.h"
 #include "YapInterface.h"
-#if STDC_HEADERS
 #include <stdlib.h>
-#endif
 #if HAVE_UNISTD_H
 #include <unistd.h>
 #endif
@@ -687,12 +690,12 @@ do_system(void)
 #if HAVE_SYSTEM
   int sys = system(command);
   if (sys < 0) {
-    return(YAP_Unify(YAP_ARG3,YAP_MkIntTerm(errno)));
+    return YAP_Unify(YAP_ARG3,YAP_MkIntTerm(errno));
   }
-  return(YAP_Unify(YAP_ARG2, YAP_MkIntTerm(sys)));
+  return YAP_Unify(YAP_ARG2, YAP_MkIntTerm(sys));
 #else
   YAP_Error(0,0L,"system not available in this configuration");
-  return(FALSE);
+  return FALSE;
 #endif
 }
 
@@ -703,6 +706,9 @@ static int
 do_shell(void)
 {
 #if defined(__MINGW32__) || _MSC_VER
+  YAP_Error(0,0L,"system not available in this configuration");
+  return(FALSE);
+#elif HAVE_SYSTEM
   char *buf = YAP_AllocSpaceFromYap(BUF_SIZE);
   int sys;
 
@@ -711,29 +717,23 @@ do_shell(void)
     return(FALSE);
   }
 #if HAVE_STRNCPY
-  strncpy(YAP_AtomName(YAP_AtomOfTerm(YAP_ARG1)), buf, BUF_SIZE);
-  strncpy(" ", buf, BUF_SIZE);
-  strncpy(YAP_AtomName(YAP_AtomOfTerm(YAP_ARG2)), buf, BUF_SIZE);
-  strncpy(" ", buf, BUF_SIZE);
-  strncpy(YAP_AtomName(YAP_AtomOfTerm(YAP_ARG3)), buf, BUF_SIZE);
+  strncpy(buf, YAP_AtomName(YAP_AtomOfTerm(YAP_ARG1)), BUF_SIZE);
+  strncpy(buf, " ", BUF_SIZE);
+  strncpy(buf, YAP_AtomName(YAP_AtomOfTerm(YAP_ARG2)), BUF_SIZE);
+  strncpy(buf, " ", BUF_SIZE);
+  strncpy(buf, YAP_AtomName(YAP_AtomOfTerm(YAP_ARG3)), BUF_SIZE);
 #else
-  strcpy(YAP_AtomName(YAP_AtomOfTerm(YAP_ARG1)), buf);
-  strcpy(" ", buf);
-  strcpy(YAP_AtomName(YAP_AtomOfTerm(YAP_ARG2)), buf);
-  strcpy(" ", buf);
-  strcpy(YAP_AtomName(YAP_AtomOfTerm(YAP_ARG3)), buf);
+  strcpy(buf, YAP_AtomName(YAP_AtomOfTerm(YAP_ARG1)));
+  strcpy(buf, YAP_AtomName(YAP_AtomOfTerm(YAP_ARG2)));
+  strcpy(buf, " ");
+  strcpy(bug, YAP_AtomName(YAP_AtomOfTerm(YAP_ARG3)));
 #endif
-#if HAVE_SYSTEM
   sys = system(buf);
   YAP_FreeSpaceFromYap(buf);
   if (sys < 0) {
-    return(YAP_Unify(YAP_ARG5,YAP_MkIntTerm(errno)));
+    return YAP_Unify(YAP_ARG5,YAP_MkIntTerm(errno));
   }
-  return(YAP_Unify(YAP_ARG4, YAP_MkIntTerm(sys)));
-#else
-  YAP_Error(0,0L,"system not available in this configuration");
-  return(FALSE);
-#endif
+  return YAP_Unify(YAP_ARG4, YAP_MkIntTerm(sys));
 #else
   char *cptr[4];
   int t;
@@ -745,17 +745,19 @@ do_shell(void)
   cptr[3]= NULL;
   t = fork();
   if (t < 0) {
-    return(YAP_Unify(YAP_ARG5,YAP_MkIntTerm(errno)));
+    return YAP_Unify(YAP_ARG5,YAP_MkIntTerm(errno));
   } else if (t == 0) {
-    t = execvp(YAP_AtomName(YAP_AtomOfTerm(YAP_ARG1)),cptr);
-    return(t);
+    t = execvp(cptr[0],cptr);
+    return t;
   } else {
     t = wait(&sys);
+    fprintf(stderr,"after wait %x:%x\n",t,sys);
     if (t < 0) {
-      return(YAP_Unify(YAP_ARG5,YAP_MkIntTerm(errno)));
+      return YAP_Unify(YAP_ARG5,YAP_MkIntTerm(errno));
     }
   }
-  return(YAP_Unify(YAP_ARG4, YAP_MkIntTerm(sys)));
+  fprintf(stderr,"after wait %x\n", sys);
+  return YAP_Unify(YAP_ARG4, YAP_MkIntTerm(sys));
 #endif
 }