Construction Universality

Like life, it seems possible to build programmable replicators as well as programmable computers. The only ingredients needed for the computers above (using the sliding block memory) are p68s and gliders. p68s can be built by colliding three gliders:
3-glider p68 synthesis

The glider "side-tracking" mechanism from Life doesn't seem to work for placing gliders on perpendicular tracks arbitrarily far away from the replicator -- the kickback reactions I know of move the track of the kicked glider inwards, towards the source of the glider causing the kickback, rather than outwards as in life. Instead, Dean suggests pushing a block to the desired track then hitting it with two gliders to form a third sideways-going glider:

conversion of block to glider

These reactions together with the glider kickbacks are sufficient to place gliders on arbitrary tracks, far away from the source of the gliders. It remains to be seen whether these gliders can all be synchronized in such a way that they put together a computer, but it seems likely that this should all work.

B35/S236 -- Cellular Automata -- D. Eppstein -- UCI Inf. & Comp. Sci.