[M2] (golly 2.7)
#R B3/S23
#C Total periodic, by Bill Gosper.
#C
#C Without the block, every cell in the universe is aperiodic.
#C
#C The block starts to delete the nth shuttling glider at time
#C 14 - 35 (-1)^n - 2 n +=A0 (1 + 12 n) 3^(2 + 5 n)
#C  28478, 13286000, 4778186074, 1537671920812,...
#C each time lurching slightly northeast.  Thus every cell in
#C the universe is eventually permanently vacant.
#C
#C Neil Bickford has "shewn that" no shuttling glider escapes
#C the reflecting walls, although all that is required is that
#C infinitely many hit the block.
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