From 55036ad9bb517d3f00dd9d049dd9fb0926d66bb3 Mon Sep 17 00:00:00 2001
From: Vitor Santos Costa <vsc@dcc.fc.up.pt>
Date: Sun, 17 May 2009 10:58:21 -0700
Subject: [PATCH] check if argument to when is bound (obs from Ulrich
 Neumerkel).

---
 pl/corout.yap | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/pl/corout.yap b/pl/corout.yap
index 8a3a11839..3adca2255 100644
--- a/pl/corout.yap
+++ b/pl/corout.yap
@@ -315,6 +315,8 @@ when(_,Goal) :-
 % $when/5 and $when_suspend succeds when there is need to suspend a goal
 %
 %
+'$when'(V, G, Done, LG0, LGF) :- var(V), !,
+	'$do_error'(instantiation_error,when(V,G)).
 '$when'(nonvar(V), G, Done, LG0, LGF) :-
 	'$when_suspend'(nonvar(V), G, Done, LG0, LGF).
 '$when'(?=(X,Y), G, Done, LG0, LGF) :-