x = 37, y = 21, rule = B3678/S34678
18bo$$18bo$$18bo$10b3o11b3o$5boo11bo11boo$10b3obboo3boobb3o$3b7o3boobb
3obboo3b7o$b4obb11ob11obb4o$b9o3b11o3b9o$5obb11ob11obb5o$oob7o3boobb3o
bboo3b7oboo$10b3obboo3boobb3o$5boo11bo11boo$10b3o11b3o$18bo$$18bo$$18b
o!
A period 29 spark oscillator.
This demonstrates that such oscillators can have large prime periods.
David I. Bell, May 1997
