>>> from matrius import MatriuDispersaIter

>>> m = MatriuDispersaIter(2, 3)

>>> set(m) == set()
True

>>> m[1, 2] = -5
>>> m[1, 1] = 9
>>> m[2, 3] = -1

>>> set(m) == set([((1, 1), 9), ((1, 2), -5), ((2, 3), -1)])
True

>>> from matrius import compta_negatius

>>> compta_negatius(m)
2