"[[0] * n] * m" is not equal to "[[0 for i in range(n)] for j in range(m)]"

	# When generate list of list, be careful of following methods, they produce different results
	>>> x = [[0] * 2] * 3
	>>> x
	[[0, 0], [0, 0], [0, 0]]
	>>> y = [[0 for i in range(2)] for j in range(3)]
	>>> y
	[[0, 0], [0, 0], [0, 0]]
	# although they look the same and are equal to each other
	>>> x == y
	True
	# but... the operation can generate different results
	>>> x[0][0] = 1
	>>> x
	[[1, 0], [1, 0], [1, 0]] # Surprise!! every list is changed
	>>> y[0][0] = 1
	>>> y
	[[1, 0], [0, 0], [0, 0]] # this is what we expected

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