:x66 (c/2 orthogonally, p4) Found by Hartmut Holzwart, July 1992. Half of this can be escorted by an HWSS. The name refers to the fact that every cell (live or dead) has at most 6 live neighbours (in contrast to spaceships based on LWSS, MWSS or HWSS). In fact this spaceship was found by a search with this restriction.

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:Xlife A popular freeware Life program that runs under the X Window System. The main Life code was written by Jon Bennett, and the X code by Chuck Silvers.

:X-pentomino Conway's name for the following pentomino, a traffic light predecessor.

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