So far as I can tell, the current state of the art for
describing the formal semantics of Scheme is described
by these two papers, which were presented at the 2008
Workshop on Scheme and Functional Programming:

Olivier Danvy.  Towards compatible and interderivable
semantic specifications for the Scheme programming
language, part I: abstract machines, natural semantics,
and denotational semantics.
http://www.schemeworkshop.org/2008/abstract.danvy.html
http://www.schemeworkshop.org/2008/paper3.pdf

Małgorzata Biernacka, Olivier Danvy.  Towards compatible
and interderivable semantic specifications for the Scheme
programming language, part II: reduction semantics and
abstract machines.
http://www.schemeworkshop.org/2008/abstract.biernacka.html
http://www.schemeworkshop.org/2008/paper4.pdf

The formal semantics that's attached to the R6RS goes off
in a somewhat different direction.  Even if you want to
continue in that direction, it would be a good idea to
understand the two papers above.

Will

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