[texhax] plain TeX: separate itmes by comme in \write environment

[Philip Taylor (RHUoL)]

Rodolfo Medina wrote:
> Hi all (La)TeX users.
>
> In (e)plain TeX: in a document of mine I'm using the \write command so to write
> a list of items in an external file and then input it in the document.  I wish
> that items were separated by a comma, apart from the last one of course.  How
> can I do that?
>
> Thanks for any help,
>
> Rodolfo
It is not exactly clear what is meant by "using the \write command so to 
write
a list of items in an external file"; can you post code, please ?

Philip Taylor