From 2f8c9c6b6c5a4c7cb08e7497901a072718d6270b Mon Sep 17 00:00:00 2001
From: Vitor Santos Costa <vsc@dcc.fc.up.pt>
Date: Wed, 22 Apr 2009 11:32:07 -0500
Subject: [PATCH] block users from catching user balls.

---
 pl/boot.yap | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/pl/boot.yap b/pl/boot.yap
index 034f2d76b..dda517ff5 100644
--- a/pl/boot.yap
+++ b/pl/boot.yap
@@ -1147,13 +1147,24 @@ throw(Ball) :-
 
 '$handle_throw'(_, _, _).
 '$handle_throw'(C, A, Ball) :-
-        % reset info 
-	(Ball \== '$abort', C = Ball ->
+        % reset info
+	('catch_ball'(Ball, C) ->
 	    '$execute'(A)
 	    ;
 	    throw(Ball)
 	).
 
+'catch_ball'('$abort', _, _) :- !, fail.
+% system defined throws should be ignored by used, unless the
+% user is hacking away.
+'catch_ball'(Ball, V) :-
+	var(V),
+	nonvar(Ball),
+	functor(Ball, Name, _),
+	atom_codes(Name, [0'$|_]), %'0
+	!, fail.
+'catch_ball'(C, C).
+
 '$run_toplevel_hooks' :-
 	nb_getval('$break',0),
 	recorded('$toplevel_hooks',H,_), !,