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+; "dynwind.scm", wind-unwind-protect for Scheme
+; Copyright (c) 1992, 1993 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.
+
+;This facility is a generalization of Common Lisp `unwind-protect',
+;designed to take into account the fact that continuations produced by
+;CALL-WITH-CURRENT-CONTINUATION may be reentered.
+
+;  (dynamic-wind <thunk1> <thunk2> <thunk3>)		procedure
+
+;The arguments <thunk1>, <thunk2>, and <thunk3> must all be procedures
+;of no arguments (thunks).
+
+;DYNAMIC-WIND calls <thunk1>, <thunk2>, and then <thunk3>.  The value
+;returned by <thunk2> is returned as the result of DYNAMIC-WIND.
+;<thunk3> is also called just before control leaves the dynamic
+;context of <thunk2> by calling a continuation created outside that
+;context.  Furthermore, <thunk1> is called before reentering the
+;dynamic context of <thunk2> by calling a continuation created inside
+;that context.  (Control is inside the context of <thunk2> if <thunk2>
+;is on the current return stack).
+
+;;;WARNING: This code has no provision for dealing with errors or
+;;;interrupts.  If an error or interrupt occurs while using
+;;;dynamic-wind, the dynamic environment will be that in effect at the
+;;;time of the error or interrupt.
+
+(define dynamic:winds '())
+
+(define (dynamic-wind <thunk1> <thunk2> <thunk3>)
+  (<thunk1>)
+  (set! dynamic:winds (cons (cons <thunk1> <thunk3>) dynamic:winds))
+  (let ((ans (<thunk2>)))
+    (set! dynamic:winds (cdr dynamic:winds))
+    (<thunk3>)
+    ans))
+
+(define call-with-current-continuation
+  (let ((oldcc call-with-current-continuation))
+    (lambda (proc)
+      (let ((winds dynamic:winds))
+	(oldcc
+	 (lambda (cont)
+	   (proc (lambda (c2)
+		   (dynamic:do-winds winds (- (length dynamic:winds)
+					      (length winds)))
+		   (cont c2)))))))))
+
+(define (dynamic:do-winds to delta)
+  (cond ((eq? dynamic:winds to))
+	((negative? delta)
+	 (dynamic:do-winds (cdr to) (+ 1 delta))
+	 ((caar to))
+	 (set! dynamic:winds to))
+	(else
+	 (let ((from (cdar dynamic:winds)))
+	   (set! dynamic:winds (cdr dynamic:winds))
+	   (from)
+	   (dynamic:do-winds to (+ -1 delta))))))