From 9b5bec99454995b40713e8af03b5853a609f2f25 Mon Sep 17 00:00:00 2001
From: David Thompson <dthompson2@worcester.edu>
Date: Thu, 30 Aug 2018 08:12:28 -0400
Subject: kernel: Add previous-scene procedure.

* starling/kernel.scm (previous-scene): New procedure.
---
 starling/kernel.scm | 13 ++++++++++---
 1 file changed, 10 insertions(+), 3 deletions(-)

diff --git a/starling/kernel.scm b/starling/kernel.scm
index 25e92b6..078b6df 100644
--- a/starling/kernel.scm
+++ b/starling/kernel.scm
@@ -58,6 +58,7 @@
             elapsed-time
             fps
             current-scene
+            previous-scene
             push-scene
             pop-scene
             replace-scene)
@@ -282,10 +283,16 @@
         (deactivate kernel)
         (close-window! (window kernel))))))
 
-(define-method (current-scene)
+(define (current-scene)
   "Return the currently active scene"
-  (and (not (null? (scenes (current-kernel))))
-       (car (scenes (current-kernel)))))
+  (match (scenes (current-kernel))
+    (() #f)
+    ((scene . rest) scene)))
+
+(define (previous-scene)
+  (match (scenes (current-kernel))
+    ((or () (_)) #f)
+    ((_ scene . rest) scene)))
 
 (define-method (push-scene (scene <scene>))
   "Make SCENE the currently active scene and remember the previously
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