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gathering
「叢」 has other readings with different meanings.
人の叢ができて何があるのか気になった。
A crowd had gathered and I wondered what was going on.
草の叢の中に小さな虫が隠れていた。
A small insect was hiding in a thicket of grass.
雲の叢が空を覆って暗くなってきた。
A mass of clouds covered the sky and it grew dark.