Every pāda scanned into laghu and guru — light and heavy syllables — by rule, not by ear. Most of the song walks in steady anuṣṭubh; watch it widen into triṣṭubh where the teaching itself grows larger.
Guru: a long vowel, or any vowel before anusvāra, visarga, or a conjunct. Everything else is laghu. छन्दस् in the glossary.
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Three padas in the received text hand-count past their metre's usual length. That is the tradition as it stands — shown here, not corrected.