def update(self, z, R2=None):
        """
        Add a new measurement (z) to the kalman filter. If z is None, nothing
        is changed.

        Parameters
        ----------

        z : np.array
            measurement for this update.

        R2 : np.array, scalar, or None
            Sqrt of meaaurement noize. Optionally provide to override the
            measurement noise for this one call, otherwise  self.R2 will
            be used.
        """

        if z is None:
            self.z = np.array([[None]*self.dim_z]).T
            self.x_post = self.x.copy()
            self._P1_2_post = np.copy(self._P1_2)
            return

        if R2 is None:
            R2 = self._R1_2
        elif np.isscalar(R2):
            R2 = eye(self.dim_z) * R2

        # rename for convienance
        dim_z = self.dim_z
        M = self.M

        M[0:dim_z, 0:dim_z] = R2.T
        M[dim_z:, 0:dim_z] = dot(self.H, self._P1_2).T
        M[dim_z:, dim_z:] = self._P1_2.T

        _, self.S = qr(M)
        self.K = self.S[0:dim_z, dim_z:].T
        N = self.S[0:dim_z, 0:dim_z].T

        # y = z - Hx
        # error (residual) between measurement and prediction
        self.y = z - dot(self.H, self.x)

        # x = x + Ky
        # predict new x with residual scaled by the kalman gain
        self.x += dot(self.K, pinv(N)).dot(self.y)
        self._P1_2 = self.S[dim_z:, dim_z:].T

        self.z = deepcopy(z)
        self.x_post = self.x.copy()
        self._P1_2_post = np.copy(self._P1_2)