m is substituted for the d of idam before a vibhakti affix.,
As इयम् ॥ This is confined to the Feminine, as the following sutra relates to the masculine; and in the Neuter, the सु is लुक् elided by स्वमोर्नपुंसक and so leaves no trace behind.,
यः सौ
Adhyāya 7 · Pāda 2 · Rule 110