उभयथ र्क्षु

This ordains an option to the last sutra, by which the रु substitution was compulsory. A word ending in म् followed by a letter of छव् class, which itself is followed by अम्, changes its final म् to र् optionally in the Rig Veda. Sometimes there is र् and sometimes म् ॥ Thus तस्मिन् त्वा दधाति or तस्मिँस् त्वा दधाति; पशूँस्ताँश्चक्रे ॥

Why do we say \in the Rig verses\. No option is allowed here ताँस्त्व खाद सुखादिताम् ॥,