interchange

Usage:
    <interchange a b c1 c2
sets c1 and c2 equal to the invertible matrices such that
c2(a0b)c1 = b0a,
where we have written a0b for the tensor
product of the matrices a and b.
Parameters:
            a,b = matrices
Output values:
            c1,c2 = invertible matrices
We first form permutation matrices corresponding to the ci,
using <interchange_permutation, then
set their degrees to match a0b.