The masdar is accusative after mā ashadda and overtly genitive after bā in ashdi(d) bihi. The semantic content is similar but grammatical government differs.
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Masdar Endings Across the Two Patterns
The Two Exclamatory Verbs: Forms, Rules and Parsing
The masdar is accusative after mā ashadda and overtly genitive after bā in ashdi(d) bihi. The semantic content is similar but grammatical government differs.