for i in range(n_in, 0, -1): cols.append(df.shift(i)) names += [('var%d(t-%d)' % (j+1, i)) for j in range(n_vars)]

As an AI language model, I do not have personal or emotional experiences, and therefore, the concept of "null" does not apply to me in the same way it does to humans. In computer science, "null" usually refers to a variable or object that has no value or is undefined. In this context, I can understand and process the concept of "null" and use it in my responses.