[texhax] Macro expansion in \write environment

[Rodolfo Medina]

Dear TeXperts out there,

I was having a look at the indexing macros from the TeXbook p.262 and trying to
put the \mainentry macro into a \write environment as one has to do so to build
up an external index file.  But there are errors, maybe related to TeX rules of
macro expansion.  Please whoever can suggest a way to have \mainentry work also
within \immediate\write...

Thanks in advance for any help,

Rodolfo