By quam diu (both clauses take the same tense; and if in past time, the perf. indic. .

http://lila-erc.eu/data/lexicalResources/LewisShort/id/LexicalSense/n47437.3 an entity of type: LexicalSense

By quam diu (both clauses take the same tense; and if in past time, the perf. indic. . 
By quam diu (both clauses take the same tense; and if in past time, the perf. indic. . 

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