it is only used for test.

if zm is not of length 2, return not initilized zm.

max = v1.max-v2.min min = v1.max-v2.min.

v1 product v2 => p[r0,r1,r2,r3] min,max = Min(p),Max(p).

max = v1.max+v2.max min = v1.min+v2.min.

if zm is not initialized, return a const null vector if zm is of type varlen and truncated, the max value is null.