This substitution of at is made in the singular number also in the Ablative -5th case after the aṅga (stems) yuṣmad and asmad,
As त्वद् and मद् ॥ For the substitution of त्व and म see 7.2.97; and त्व and म + अत् and मत् by (VI. I. 97).,
एकवचनस्य च
Adhyāya 7 · Pāda 1 · Rule 32