'Get a cached matrix by name and format.
Parameters
name : str
A descriptive name for the matrix, like "identity2".
format : str
The format desired (\'sympy\', \'numpy\', \'scipy.sparse\')'

def get_matrix(self, name, format):

    m = self._cache.get((name, format))
    if (m is not None):
        return m
    raise NotImplementedError(('Matrix    with    name    %s    and    format    %s    is    not    available.' % (name, format)))
