hashmaps can now be printed and keys deleted

bin/pre.slime

@@ -7,6 +7,16 @@
 (define-syntax (pe expr)
   `(print ',expr "evaluates to" ,expr))
 
+(define the-empty-stream ())
+
+(define (stream-null? s) (if s t ()))
+
+(define-syntax (delay expr)
+  `(,lambda () ,expr))
+
+(define (force promise)
+  (promise))
+
 (define-syntax (when condition . body)
   :doc "Special form for when multiple actions should be done if a
 condition is true.