(x//2)//3 vs x//6 differs at: []    (x//-2)//3 vs x//-6 differs at: []    (x//2)//-3 vs x//-6 differs at: [-47, -41, -35, -29, -23, -17] ...
the identity holds whenever the outer divisor is positive, even if the inner one is negative. the rule's guard (both positive) is stricter than it needs to be. a negative outer divisor breaks it.
