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さかのぼる， 遡る， 溯る， 逆上る， 泝る
1. to go upstream, to go up (a river)
2. to go back (to the past or origin), to date back (to), to trace back (to), to be retroactive (to)
(沿河流)上溯；回溯(历史等)
は
1. indicates sentence topic
2. indicates contrast with another option (stated or unstated)
3. adds emphasis
指示句子的主题；表示与另一选项（无论是否明确提及）形成对比；表示强调
に
1. at (place, time), in, on, during
2. to (direction, state), toward, into
3. for (purpose)
4. because of (reason), for, with
5. by, from
6. as (i.e. in the role of)
7. per, in, for, a (e.g. "once a month")
8. and, in addition to
9. if, although
tbd
