Bi-ghāʾibin is genitive in form owing to the extra bāʾ, but occupies the accusative predicate slot of laysa.
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A Genitive Form May Have Another Syntactic Slot
Comprehensive Review: Applied Arabic Parsing
Bi-ghāʾibin is genitive in form owing to the extra bāʾ, but occupies the accusative predicate slot of laysa.