[R6RS] cost of letrec* semantics for internal definitions

[R. Kent Dybvig]

> Out of curiosity concerning current practice, I ran a
> very simple test case using the interactive REPL of the
> usual suspects.  SCM and Scheme48 detected the violation
> of the letrec restriction.  Gambit, Larceny, and Chez
> Scheme 6.1 (Sparc) detected it, but only by accident:
> the + procedure complained about a non-numeric argument.
> The following systems did not detect the violation at
> all: Bigloo, Chicken, Kawa, MIT Scheme, MzScheme, and
> Petite Chez 7.0a.

By Version 7.0a, (Petite) Chez Scheme uses letrec* semantics for internal
definitions and so for them will detect violations of the letrec*
restriction.  It will detect violations of the letrec restriction when
letrec is actually used.

Kent