Formatting index entry numbers

[Mark Summerfield]

Hi,

Does anyone know how to format the number (not the text) of an index
entry? I'm working on a book and for each index entry, one entry is the
"main" entry, and the others are subordinate, for example:

    My Term, 34, 44-55, _61_, 72

Here page number 61 should appear in bold. I can't see a way of doing
it. Is it possible, or would new symbols be required (e.g. @PrimeIndex,
@PrimeSubIndex or similar, with a @PrimeIndexFormat setting)?

Thanks.

-- 
Mark.