Dear Jim,
If we take a look at the occurrences of yathayida.m there are many passages
that cannot be construed grammatically if we assume that ida.m has to be in
agreement with a possible noun. ida.m is no doubt used adverbially as
suggested in CPD s.v. ayam, as is also the case with nayida.m. I would
suggest that yathayida.m is < yathaa + onglide /y/ before /i/ of ida.m used
adverbially.
Ole
<Looking at other similar glosses I came across a few that had 'yathaa ime'
which shows that 'ida.m' can also stand for the plural and presumably
differences in gender too>
These, I believe, are examples of commentarial ingenuity.
<A gloss on 'nayida.m' at Ps-p.t I 203 (Be) explicitly states that the
'ida.m' in a similar phrasing is a particle:
"nayidanti ettha idanti nipaatamatta.m."
This is a good example of a correct analysis. Think of similar examples like
eka.m idaaha.m etc.