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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def transform(self, X_all, y=None):
 """Subsample several time series. Parameters X_all : list(np.ndarray) List of feature time series Returns ------- features : list(np.ndarray), length = len(X_all) The subsampled trajectories. """

        if self._sliding_window:
            return [X[k::self._lag_time] for k in range(self._lag_time) for X in X_all]
        else:
            return [X[::self._lag_time] for X in X_all]