Special variables to relax boxing

[Stefan Israelsson Tampe]

Hi,

I wouldl like to start a discussion of introducing a way to mark
variables as special, and by that mean that set! variables does not
nessesary get boxed. This is a deviation from the scheme standard but
it's pretty useful if one want to set! local variables but make sure
that the state of the function can be stored in a continuation and
later restart at the same state.

This patch introduces a macro (mark-as-special var), this will mark
var as a special variable in the above meaning, please look at the
example in special.scm for more info.

There is a preleminary patch following the document against stable-2.0
that we can use as a discussion point for know about this feature.

Best Greetings
Stefan

special.scm
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special.diff
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