1 files changed, 52 insertions, 0 deletions
diff --git a/ps07_amb/stack-queue.scm b/ps07_amb/stack-queue.scm
new file mode 100644
index 0000000..71f7c0e
--- /dev/null
+++ b/ps07_amb/stack-queue.scm
@@ -0,0 +1,52 @@
+;;;; Simple stack&queue Abstraction
+
+(declare (usual-integrations))
+
+(define-record-type <stack&queue>
+    (%make-stack&queue front back)
+    stack&queue?
+  (front stack&queue-front set-stack&queue-front!)
+  (back stack&queue-back set-stack&queue-back!))
+  
+
+(define (make-stack&queue)
+  (%make-stack&queue '() '()))
+
+(define (stack&queue-empty? stq)
+  (not (pair? (stack&queue-front stq))))
+
+(define (stack&queued? stq item)
+  (memq item (stack&queue-front stq)))
+
+(define (push! stq object)
+  (if (pair? (stack&queue-front stq))
+      (set-stack&queue-front! stq
+        (cons object (stack&queue-front stq)))
+      (begin
+	(set-stack&queue-front! stq
+	  (cons object (stack&queue-front stq)))
+	(set-stack&queue-back! stq
+	  (stack&queue-front stq))))
+  unspecific)
+
+(define (add-to-end! stq object)
+  (let ((new (cons object '())))
+    (if (pair? (stack&queue-back stq))
+	(set-cdr! (stack&queue-back stq) new)
+	(set-stack&queue-front! stq new))
+    (set-stack&queue-back! stq new)
+    unspecific))
+
+(define (pop! stq)
+  (let ((next (stack&queue-front stq)))
+    (if (not (pair? next))
+	(error "Empty stack&queue -- POP"))
+    (if (pair? (cdr next))
+	(set-stack&queue-front! stq (cdr next))
+	(begin
+	  (set-stack&queue-front! stq '())
+	  (set-stack&queue-back! stq '())))
+    (car next)))
+
+
+