Karnatakabhashabhushana 3.13

After [a stem] ending in short a, the augment n- [is inserted] prior to gaḷ.

Prior to gaḷ, following a word-form ending in short a, the augment n- occurs.

gajaṅgaḷ,

hayaṅgaḷ.

Because of the governing rule of diversity (bahuḷa), [it is] optional in some cases:

maraṅgaḷ,

maragaḷ;

koḷaṅgaḷ,

koḷagaḷ.

Why [is it said] "after [a stem] ending in short a"?

maḍigaḷ,

maḍugaḷ,

manegaḷ,

ūrgaḷ,

sālgaḷ.