>>> L = [[k for k in range(m)] for m in range(1,6)]
>>> L
[[0], [0, 1], [0, 1, 2], [0, 1, 2, 3], [0, 1, 2, 3, 4]]
>>> fusion=[x for SL in L for x in SL]
>>> fusion
[0, 0, 1, 0, 1, 2, 0, 1, 2, 3, 0, 1, 2, 3, 4]