From cdf3b66fcc24069faf53f8d0f3d0553cd06ef30f Mon Sep 17 00:00:00 2001
From: Spencer Janssen <sjanssen@cse.unl.edu>
Date: Tue, 8 May 2007 17:21:16 +0200
Subject: Remove unsafe use of head

darcs-hash:20070508152116-a5988-16a82d62661a5ccd0a95f88ced57cd3e9df5fd67
---
 StackSet.hs | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/StackSet.hs b/StackSet.hs
index 38baca2..e0210f1 100644
--- a/StackSet.hs
+++ b/StackSet.hs
@@ -188,7 +188,8 @@ raiseFocus k w = case M.lookup k (cache w) of
 promote :: (Integral i, Ord a) => StackSet i j a -> StackSet i j a
 promote w = maybe w id $ do
     a <- peek w -- fail if null
-    let w' = w { stacks = M.adjust (\(f, s) -> (f, swap a (head s) s)) (current w) (stacks w) }
+    (f, xs@(x:_)) <- M.lookup (current w) (stacks w)
+    let w' = w { stacks = M.insert (current w) (f, swap a x xs) (stacks w) }
     return $ insert a (current w) w' -- and maintain focus (?)
 
 -- | Swap first occurences of 'a' and 'b' in list.
-- 
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