A recursive definition of narrowings
1. An argument ? has no narrowings
2. An argument {A} ? ? has an argument {B} ? ? as a narrowing if B is a narrowing of A
3. An argument {A1, A2, …, An} ? ? has an argument {B1, B2, …, Bm} ? ? (not equal to the argument itself) as a narrowing if the arguments {B1} ? ?, {B2} ? ?, …, and {Bm} ? ? are equal to or narrowings of (different) arguments {A1} ? ?, {A2} ? ?, …, and {An} ? ?
? Note: The CumulA-definition slightly differs from this one in that it (unnecessarily) demands that for coordinated arguments the argument's final conclusion is supported by less reasons.