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+;"sc4sc3.scm" Implementation of rev4 procedures for rev3.
+;Copyright (C) 1991 Aubrey Jaffer
+;
+;Permission to copy this software, to redistribute it, and to use it
+;for any purpose is granted, subject to the following restrictions and
+;understandings.
+;
+;1.  Any copy made of this software must include this copyright notice
+;in full.
+;
+;2.  I have made no warrantee or representation that the operation of
+;this software will be error-free, and I am under no obligation to
+;provide any services, by way of maintenance, update, or otherwise.
+;
+;3.  In conjunction with products arising from the use of this
+;material, there shall be no use of my name in any advertising,
+;promotional, or sales literature without prior written consent in
+;each case.
+
+;;;; peek-char, number->string, and string->number need to be written here.
+
+;;; APPEND, +, *, -, /, =, <, >, <=, >=, MAP, and FOR-EACH need to
+;;; accept more general number or arguments.
+
+(define (list? x)
+  (let loop ((fast x) (slow x))
+    (or (null? fast)
+	(and (pair? fast)
+	     (let ((fast (cdr fast)))
+	       (or (null? fast)
+		   (and (pair? fast) 
+			(let ((fast (cdr fast))
+			      (slow (cdr slow)))
+			  (and (not (eq? fast slow))
+			       (loop fast slow))))))))))