Halliday & Matthiesen (2014: 475):
Non-finite hypotactic extending clauses cover both (a) addition and (b) variation. Two subtypes are absent from the non-finite system: ‘negative additive’ addition and ‘alternative’ variation. The non-finite form of hypotactic extending is an imperfective clause; for example (structure α +β):||| We used to go away at the weekend, || taking all our gear with us. |||