Karnatakabhashabhushana 8.17

Before [an ending] beginning with d, the augment iD is diverse.

When a set of endings beginning with the phoneme d follows, the augment i occurs diversely.

meccidaṁ,

meccidapaṁ,

meccidar,

[and] meccidapar.

Because of the word bahuḷam ("diverse"), it occurs elsewhere too:

urcitu,

urcittu,

bercitu,

[and] bercittu.

In some cases it is something else altogether:

naḍedattu,

oḍedaṁ,

naḍedaṁ,

kaḷedaṁ,

[and] beḷedaṁ.