Mutated worm

See the problem 371 by Philippe. We slightly modify the evolution code in that it creates m copies instead of (m+1) at m_th step. So the evolution of [1,1] now becomes like the following: 1: [1, 1] 2: [1, 0, 1, 0] 3: [1, 0, 1] 4: [1, 0, 0, 0, 0, 0] 5: [1, 0, 0, 0, 0] 6: [1, 0, 0, 0] 7: [1, 0, 0] 8: [1, 0] 9: [1] 10: [0, 0, 0, 0, 0, 0, 0, 0, 0, 0] 11: [0, 0, 0, 0, 0, 0, 0, 0, 0] 12: [0, 0, 0, 0, 0, 0, 0, 0] 13: [0, 0, 0, 0, 0, 0, 0] 14: [0, 0, 0, 0, 0, 0] 15: [0, 0, 0, 0, 0] 16: [0, 0, 0, 0] 17: [0, 0, 0] 18: [0, 0] 19: [0] 20: [] At which step does the worm W₁ = [1,2] turn into []? Answer format: rightmost 20 digits [My timing < 1s]