The word anu 'near to' is optionally compounded with that word, nearness to which is indicated by the particle and the resulting compound is avyayībhāva,
As, अनुवनमशनिर्गतः 'the thunder bolt fell near the forest.'
Why do we say अनु ? Observe वनं समया 'near the forest.'
Why do we say 'when compounded with the word, nearness to which is indicated'? Observe वृक्षमनु विद्योतते विद्युत् 'it lightens towards the tree.'
Though this compounding was valid even by sūtra 6, because nearness is mentioned there also; it is repeated here in order to show that an Avyayibháva compound mentioned here is an optional compound after all, and not an invariable compound like others.,