There is reduplication of yar , i.e. all the consonants except ha after the letters r and h following a vowel.,
The word यर् of last sutra is understood here. According to others, the वा is also understood, and this is an optional rule. Thus अर्क्कः, मर्क्कः, ब्रह्म्मा, अपह्न्नुते ॥
Why do we say अचः 'following a vowel'? Observe ह्नुते, ह्मलयति ॥,