Ceci n'est pas une value (was Re: [R6RS] "void" or "unspecific")

[William D Clinger]

> Related to this, the last two or three status reports have specified 
> that R6RS will have "read-write invariance for every datum".  Our final 
> vote on #3 today was to not add #!unspecified as syntax.  How, then, 
> will read-write invariance work in a case such as:
> 
>    (write (list (unspecified)))

The word "datum" means one of the external representations
that are generated by the nonterminal <datum> in the formal
syntax of Scheme.  Since the value returned by (unspecified)
has no external representation (aside from implementation-
specific extensions), it is not a datum.

Will