How should “li-ʿillatin takūnu hunāk” be understood?

The Compilation Method: Narrator Tiers, Justified Repetition and Abridgement

• Mentioning a second chain establishes a disqualifying defect in the first
• A particular critical reason calls for comparison, to be assessed through evidence
• Every second chain necessarily serves as a sound corroborating route
• The critical reason can concern only textual meaning, not transmission