def sum(self, axis=None, dtype=None, out=None):
return N.ndarray.sum(self, axis, dtype, out, keepdims=True)._collapse(axis)<EOL>
Returns the sum of the matrix elements, along the given axis.

Refer to `numpy.sum` for full documentation.

See Also
--------
numpy.sum

Notes
-----
This is the same as `ndarray.sum`, except that where an `ndarray` would
be returned, a `matrix` object is returned instead.

Examples
--------
>>> x = np.matrix([[1, 2], [4, 3]])
>>> x.sum()
10
>>> x.sum(axis=1)
matrix([[3],
        [7]])
>>> x.sum(axis=1, dtype='float')
matrix([[ 3.],
        [ 7.]])
>>> out = np.zeros((1, 2), dtype='float')
>>> x.sum(axis=1, dtype='float', out=out)
matrix([[ 3.],
        [ 7.]])