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Now I have some questions and a "wish list". Questions: how do upward
continuations (the ones created with CALL-WITH-CURRENT-CONTINUATION),
which can be multiply invoked, interact with dynamic binding and
UNWIND-PROTECT? E.g., if a continuation is created in the (dynamic)
scope of an unwind-protect, do the cleanup forms get executed only the
first time the continuation is invoked, or every time, or what?
MIT Scheme has a procedure (rather than a special form) called
DYNAMIC-WIND, which is a generalization of UNWIND-PROTECT.
(lambda () <entry>)
(lambda () <body>)
(lambda () <exit>))
is similar to
(let ((val <body>))
except that <exit> is executed whenever <body> is "escaped", and
<entry> is executed whenever <body> is (re)entered.