The affix yuc comes, in the sense of 'the agent having such a habit etc.' after intransitive verbs as are ātmanepadi and begin with a consonant and are anudātta i.e. have a gravely accented vowel as as indicatory.,
As वर्त्तनः 'being'; वर्द्धनः 'growing'.
Why do we say 'Anudattet'? Observe भविता.
Why do we say 'beginning with a consonant'? Observe एधिता, which though Atmanepadi, takes तृन्.
Why have we used the term आदि 'beginning' in the sutra? Observe जुगुप्सनः 'censuring'; मीमांसनः 'an investigator'. For without the word आदि, the rule would have stood thus:- अनुदात्तेतश्च हलः which by (I. I. 72.) would mean 'the affix युच् comes after the verbs which end with a consonant'; so that even after एध् we shall have युच्; and there would be no युच् after जुगुप्स which ends with a vowel.
This rule will not apply if the root is transitive; as वसिता वस्त्रं ।,