@staticmethod
def qr(a):
    """Compute the qr factorization of a matrix.

        Factor the matrix `a` as *qr*, where `q` is orthonormal and `r` is
        upper-triangular.

        Parameters
        ----------
        a : tensor, shape (M, N)
            Matrix to be factored.

        Returns
        -------
        Q, R : tensor
        """
    raise NotImplementedError
