Nā chhiá ta nā

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Nā chhiá ta nā (Eng-gí: if and only if; iff) sī chi̍t khoán lô-chek tiâu-kiāⁿ, khah chheng-chhó lâi kóng tiō sī nā-sī kiam ta-ū nā-sī, sī piau-chún bu̍t-chit tiâu-kiāⁿ (material conditional) "ta nā" (tùi-tâng "nā ... tiō") kap i-ê tó-hoán "nā" cho͘-ha̍p--khí-lâi tùi nn̄g hāng tîn-su̍t ê siang-tiâu-kiāⁿ lô-chek liân-chiap (biconditional logical connective) koan-hē.

Pún tiâu-kiāⁿ kong-tēng ê ì-sù sī piáu-sī i só͘ liân-chiap ê siang-pêng tîn-su̍t, nā m̄-sī siâng-pêng lóng sī chiàⁿ-si̍t--ê, tiō sī siâng pêng lóng sī m̄-tio̍h--ê; he̍k-chiá kóng kî-tiong chi̍t hāng tîn-su̍t ê chiàⁿ-si̍t, sī ài óa-khò lēng-gōa chi̍t hāng ê chiàⁿ-si̍t.

Chit-ê tiâu-kiāⁿ kap pì-iàu kiam chhiong-chiok tiâu-kiāⁿ sī sio-siâng ê ì-sù. Nā kóng P nā chhiá ta nā Q, ì-sù kap Q tùi P pì-iàu kiam chhiong-chiok sī sio-kâng--ê; mā kap P sī kap Q téng-ta̍t (equivalent) kâng ì-bī.