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I don't have enough context to write an accurate overview of "678." The context provided only indicates it's a year, but without additional information about what specific event, work, person, or topic "678" refers to, I cannot provide a reliable 2-sentence explanation that meets your accuracy requirements.
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Article
Pope Agatho I (678–681)
Year 678 (DCLXXVIII) was a common year starting on Friday of the Julian calendar. The denomination 678 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.