[R6RS] cost of letrec* semantics for internal definitions

[R. Kent Dybvig]

> How, then, could this matter be "easily addressed by
> allowing such programs to be rejected before execution"?
> Do you have a statically decidable version of the letrec*
> restriction you'd like to propose for the R6RS?

A statically undecidable restriction would be a problem only if we were to
*require* programs that violate the restrictions to be rejected prior to
execution.  Simply *allowing* programs that violate the restriction to be
rejected before execution would be sufficient to prevent people from
considering such programs valid programs for which they can count on
certain run-time behavior, i.e., exceptions being raised.

Kent