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

[Rodolfo Medina]

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