The final i and the u of a word with the exception of pīlu are lengthened before vaha,
Thus ऋषीवहम्, कपीवहम्, मुनीवहम् ॥ Why do we say 'ending in इक् vowels'? Observe पिण्डवहम् ॥ Why not of पीलु? Observe पीलुवहम् ॥
Vart:- It should be stated \with the exception of पीलु and the rest.\ As दारुवहम् ॥,