Set of matrices

I have lots of matrices (as result of rotations, etc.), but I would be sure to store them only once. I thought about using a set :

print set([np.matrix([[0, 0],[0, 1],[1, 1],[2, 1]]), 
 np.matrix([[0, 0],[1, 0],[1, -1],[1, -2]])])

Unfortunately, I get :

TypeError: unhashable type: 'matrix'

It's sad because a set would be exactly the right data type to avoid repetitions, and be able to use union, intersection, etc.

How to work with sets of matrices in Python?