adj. 25B6; adjective OBLIGATORY, mandatory, required, requisite, necessary, essential; imperative, unavoidable, enforced, demanded, prescribed.
optional.
adj. 25B6; adjective OBLIGATORY, mandatory, required, requisite, necessary, essential; imperative, unavoidable, enforced, demanded, prescribed.
optional.
Concise Oxford thesaurus English vocabulary. Краткий оксфордский словарь английского языка тезаурус. 2012