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to be present (esp. during a birth); to be witness to; to attend
出産に立ち会うことができた。
I was able to be present at the birth.
検査官が検証に立ち会っている。
An inspector is present at the verification.
歴史的な調印式に立ち会えたのは一生の思い出だ。
Being able to witness the historic signing ceremony is a memory for a lifetime.