<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:
 def _ar_matrix(self):
 """ Creates the Autoregressive matrix for the model Returns X : np.ndarray Autoregressive Matrix """


        X = np.ones(self.data_length-self.max_lag)

        if self.ar != 0:
            for i in range(0, self.ar):
                X = np.vstack((X,self.data[(self.max_lag-i-1):-i-1]))

        return X