>> an overwrite argument
>>
>> It is worthwhile to add this to the SRFI 170 API, as, say, an
>> optional argument, defaulting to overwrite?
>
> I've never seen anyone use it... People always want to overwrite the old value.
> Therefore I suggest leaving out the option from the Scheme API.

Another option would be something like:

(get-environment-variable name [default-value])
(update-environment-variable! name proc [default-value])

Where the signature of proc is

(lambda (old-value) ...) => new-value

and old-value is the old value of the envar (of default-value if the
envar is not yet defined).

If default-value is not given, it defaults to #f.

This is probably also overkill?