From 805ad7f9586556e9fedb5a095bd468a0306de039 Mon Sep 17 00:00:00 2001
From: Vitor Santos Costa <vsc@Vitors-MacBook-Pro.local>
Date: Mon, 9 Jan 2012 23:46:17 +0000
Subject: [PATCH] update length to use skip_list/3.

---
 pl/sort.yap | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/pl/sort.yap b/pl/sort.yap
index 0d5491bd0..c712686df 100644
--- a/pl/sort.yap
+++ b/pl/sort.yap
@@ -24,7 +24,12 @@
 
 %   length of a list.
 
-length(L,M) :- ( var(M) -> '$$_length1'(L,M,0) ; '$$_length2'(L, M) ).
+length(L, M) :-
+	  '$skip_list'(M0, L, R),
+	  ( R == [] -> M = M0 ;
+	    var(R) -> '$$_length1'(R, M, M0) ;
+	    '$do_error'(type_error(list,L),length(L,M))
+	  ).
 
 '$$_length1'([], M, M).
 '$$_length1'([_|L], O, N) :-