In the room of ṛ + a of the case-affix as of the Ablative and Genitive singular, the single substitute is the letter short u,
Thus होतृ + अस् = होतुर्स् (The उ must always be followed by र् (I. I. 51) though this उ is not the substitute of ऋ only, but of ऋ + अ conjointly: on the maxim that a substitute which replaces two, both shown in the genitive case, as ऋतः and अकारस्य in this sutra, gets the attributes of every one of these separately as the son C of a father A and mother B (though both conjointly produce him) may be called indifferently the son of A or the son of B. So the उ may be called the substitute of ऋ or अ) II The final स् is then elided by 8.2.24 and we have होतुर् = होतुः ॥ Thus होतुरागच्छति, होतुः स्वम् ॥,