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In page Unua Libro:

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In 1905, Zamenhof reproduced much of the content of Unua Libro in Fundamento de Esperanto, which he established as the only obligatory authority over Esperanto in the Declaration of Boulogne at the first World Esperanto Congress later that year. However, in his 1888 Aldono al la Dua Libro (Supplement to the Second Book), he officially altered the spelling of the suffixes of the temporal correlatives (when, then, always, sometimes, never) from -ian to -iam, which rendered the Esperanto of Unua Libro slightly outdated.[citation needed]