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+;;;"promise.scm" promise for force and delay
+;;; From Revised^4 Report on the Algorithmic Language Scheme
+;;; Editors: William Clinger and Jonathon Rees
+;
+; We intend this report to belong to the entire Scheme community, and so
+; we grant permission to copy it in whole or in part without fee.  In
+; particular, we encourage implementors of Scheme to use this report as
+; a starting point for manuals and other documentation, modifying it as
+; necessary.
+
+(define promise:force (lambda (object) (object)))
+
+(define make-promise
+  (lambda (proc)
+    (let ((result-ready? #f)
+	  (result #f))
+      (lambda ()
+	(if result-ready?
+	    result
+	    (let ((x (proc)))
+	      (if result-ready?
+		  result
+		  (begin (set! result-ready? #t)
+			 (set! result x)
+			 result))))))))
+
+;;; change occurences of (DELAY <expression>) to
+;;; (MAKE-PROMISE (LAMBDA () <expression>))
+;;; and (define force promise:force)