After 1. dvi 2. antara and prepositions, long ī is the substitute of ap,
By 1.1.54, ई replaces the first-letter of अप् viz अ only. Thus द्वीपः, अन्तरीपः, नीपम्, वीपम्, समीपम् ॥ The samasanta अ is added by 5.4.74.
Vart:- Prohibition should be stated with regard to समाप ॥ समापं नाम देवयजनम् ॥ Others say after a Preposition in अ, the long ई substitution does not take place. Thus प्रापम्, परापम् ॥ The word upasarga strictly speaking denotes adverbs, and cannot be applied to nouns like अप्; here however, this word is used in a loose sense in the sutra. It means प्र &c.,