Today we will look at the application of 6-1-77 इको यणचि briefly and provide a couple of examples to demonstrate its usage. The वृत्तिः for this sutra from Laghu Siddhanta Kaumudi goes as:
इकः स्थाने यण् स्यादचि संहितायां विषये When an अच् letter follows in close proximity (संहितायाम्), then in place of a इक् letter a यण् letter is substituted.

The pratyahara’s इक् and यण् are as follows:
इक् = इ उ ऋ ऌ
यण् = य् व् र् ल्

Here are a couple of examples from Bhagavad Gita that use this sandhi rule:
Example 1 – Excerpt from Bg1-10
पर्याप्तं त्विदमेतेषाम्
तु + इदम्
Here उ is an इक् letter and the अच् इ follows, therefore उ gets replaced व्
त् व् इदम् = त्विदम्

Example 2 – Excerpt from Bg6-2
न ह्यसन्न्यस्तसङ्कल्पः
हि + असन्न्यस्तसङ्कल्पः
The इ is an इक् and the अच् अ follows, therefore इ gets replaced य्,
ह् य् असन्न्यस्तसङ्कल्पः = ह्यसन्नस्तसङ्कल्पः

Questions:

1. Where, if any, is the sutram 6-1-77 इकोयणचि applied in the following snippets:
ब्रवीमि भो इत्यृचः
तद् दूरे तद्वन्तिके
घस्लादेशः

2. Why doesn’t the वृत्तिः for 6-1-77 इको यणचि provide a one-to-one mapping between the स्थानी (letter to be replaced) and the अादेशः (the replacement)? For e.g it should’ve said “replace इ with य् , उ with व्” etc. The वृत्तिः for another sutra 6-1-78 एचोऽयवायावः explicitly states how each स्थानी should be replaced with the corresponding अादेशः by using the परिभाषासूत्रम् 1-3-10 यथासंख्यमनुदेशः समानाम् But nothing of that sort is mentioned in 6-1-77 इको यणचि What could be the reason behind this?

3. Is there a scenario when this rule cannot be applied? If so, can you quote the Pannini sutra and a few examples?