trivalI in compounds

[Oliver Fallon]

Dear List Members,

Can anyone help finding a reference in Paninian literature for the
lengthening of the final vowel of trivali to trivalI in compounds?

MW says "f. in comp. the 3 folds over a woman's navel (regarded as a beauty)
VarBṛS. lxx , 5 Ṛitus. GarP."

I have looked in P. 6.3.114-139, which appears to be the relevant section,
but may have missed something.

With thanks,
Oliver