[plt-scheme] ifndef in PLT?

> > Could you suggest a more elegant solution?
>
> See `namespace-variable-value'.

It did the trick. Thank you for your advice.

> By the way, why do you really need this?
>

Well, as far as I understand, the built-in support for `string-upcase' and
`string-downcase' was introduced to mzscheme since PLT v. 299.200. I have a
piece of code that uses these functions and that has to work for PLT
versions [204, the latest one]. Since PLT module system prohibits multiple
definitions of an identifier, I thus have to not-define `string-upcase' and
`string-downcase' if they are already defined - and provide their definition
otherwise.

Thank you for your help.
Dmitry