Dear Bhante Sobhana,

> A.t.thahaakaarehi is a compound term in third case, made by dropping the
> case of the first word: a.t.tha + hi + aakaara + hi. Modern grammarians
> consider 'ehi' as an instrumental case. According to kaccaayana, the case
> ending is not ehi, but hi. The commentary on sa.yutta nikaaya, Mahaavagga
> .tikaa says a.t.thahaakaarehi means a.t.thahi kaara.nehi.
>
> If the term is a.t.thaakaara, then it is a case of sandhi word: a.t.t.ha +
> aakaara

I take "a.t.thahaakaarehi" as two separate words, i.e., a.t.thahi aakaarehi.
The elision of the 'i' at the end of "a.t.thahi" can be easily accounted for
by a general sandhi rule in Kaccaayana (Kc 12: saraa sare lopa.m) which
ordains that vowels are elided before a following vowel. However this is
only the first of a set of rules dealing with combinations of vowels

Best wishes,

Jim