Expectation of X_t given X_{t-1}=xp

def predmean(self, xp): 
        """ Expectation of X_t given X_{t-1}=xp
        """
        out = np.empty_like(xp)
        for i in range(3):
            out[:, i] = xp[:, i] + self.delta * xp[:, i+3]  # position
            out[:, i+3] = xp[:, i+3]                      # velocity 
        return out