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increasingly; more and more; decreasingly (when declining); less and less
彼の健康状態は益益良くなってきています。
His health condition is getting better and better.
研究は益益進んで成果が期待されている。
The research is progressing more and more, and results are expected.
益益寒くなってきたね。
It's getting colder and colder, isn't it?