وَهُوَ أَيْضًا قَائِمٌ مَقَامَ السَّبَبَيْنِ: الْجَمْعِيَّةِ وَلُزُومِهَا وَامْتِنَاعِ أَنْ يُجْمَعَ مَرَّةً أُخْرَى جَمْعَ التَّكْسِيرِ، فَكَأَنَّهُ جُمِعَ مَرَّتَيْنِ.
Translation: This pattern also stands in place of two causes: its plurality and the fixed character of that plurality, which prevents making another broken plural from it—as though it had been pluralized twice.
The author uses this reasoning for this particular ultimate-plural pattern, not for plurals in general.
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Diptote Causes — Ultimate Broken-Plural Patterns
وَهُوَ أَيْضًا قَائِمٌ مَقَامَ السَّبَبَيْنِ: الْجَمْعِيَّةِ وَلُزُومِهَا وَامْتِنَاعِ أَنْ يُجْمَعَ مَرَّةً أُخْرَى جَمْعَ التَّكْسِيرِ، فَكَأَنَّهُ جُمِعَ مَرَّتَيْنِ.
Translation: This pattern also stands in place of two causes: its plurality and the fixed character of that plurality, which prevents making another broken plural from it—as though it had been pluralized twice.
The author uses this reasoning for this particular ultimate-plural pattern, not for plurals in general.