ā is substituted for the final of yuṣmad and asmad before the case-ending of the Accusative -2nd case.,
As त्वाम्, माम्, युवाम्, आवाम्, युष्मान् and अस्मान् ॥ This rule applies to endings which are even substitutes, otherwise the last rule would have been sufficient. See 7.2.28, 7.2.29, and 7.2.92 and 7.2.97.,