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