According to the opinion of śākalya , the simple vowel followed by ṛ retains its original form, and if the vowel is long, it is shortened.,
Thus खट्वा ऋश्यः, कुमारि ऋश्यः, होतृ ऋश्यः ॥ Why do we say when followed by ऋ? Observe, खट्वेन्द्रः ॥ Why do we say \the simple vowels (अकः)\? Observe वृक्षावृश्यः (वृक्षा + ऋश्य) ॥ This rule applies even when the vowels are homogenous (which were excepted by the last rule), and it is not confined to इक् vowels as the last, but applies to अ and आ also.,