أَسْوَدُ: originally adjectival; its verb-like pattern contributes to diptote status.
أَرْبَعٍ: in the numerical construction cited by the book, this is fully declinable because adjectival function is not original.
An outer resemblance to the أَفْعَل pattern is not enough: the word's original lexical function and the rule's conditions matter.
Treating both alike would erase the book's explicit restriction.
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Causes of Diptote Status — ʿAdl and Waṣf
أَسْوَدُ: originally adjectival; its verb-like pattern contributes to diptote status.
أَرْبَعٍ: in the numerical construction cited by the book, this is fully declinable because adjectival function is not original.
An outer resemblance to the أَفْعَل pattern is not enough: the word's original lexical function and the rule's conditions matter.
Treating both alike would erase the book's explicit restriction.