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to write (to someone)
祖母に近況を書き送った。
She wrote to her grandmother, updating her on her recent situation.
先生に訪問の旨を書き送って知らせた。
He wrote ahead to his teacher, informing him of his visit.
毎年、遠方の友人たちに近況を書き送るのが彼の習慣だ。
Every year, it is his custom to write to his distant friends, updating them on his life.