> On Jun 27, 2021, at 3:45 PM, Marc Nieper-Wißkirchen <xxxxxx@gmail.com> wrote:
>
> Using cond-expand is certainly a good option because it is already standardized and well-known. As neither R6RS nor R7RS support it at the outmost level of a top-level program,

By that I assume you mean that the cond-expand body is defined in R7RS to be a sequence of expressions?  So the R7RS does not specify if definitions, (import …), etc are allowed in the body.  I would be surprised if a Scheme system dissallowed definitions in cond-expands that are not in expression position, so a pragmatic point of view could be adopted: for those Scheme systems a more involved wrapping is required by the scheme-script-ce dispatcher if those systems are to be supported.  Moreover, it is very likely that cond-expand can verify the identity of the Scheme implementation and act in the appropriate way for that system.

Marc