x0 = 0 x1 = x0 x2 = x1 x3 = x2 x4 = x3 x5 = x4 x6 = x5 x7 = x6 x8 = x7 x9 = x8 x10 = x9 x11 = x10 x12 = x11 x13 = x12 x14 = x13 x15 = x14 x16 = x15 x17 = x16 x18 = x17 x19 = x18