These vanishing reactions can be used as sparks for other reactions, or to form glider logic circuits. There are also several ways of colliding two gliders so that only one glider results:
These reactions can be used to position gliders for another reaction, to synchronize glider logic streams, or to nondestructively test for the presence of a glider. The reactions in which two head-on gliders produce a sideways glider should be especially useful in building glider salvos by setting up pairs of guns on opposite sides of the salvo track; this reaction is also used in my delay line construction.
Less useful curiosities are reactions in which two head-on gliders turn into two sideways gliders, and in which four gliders become six or six become eight. (However the first reaction could be used for duplicating glider streams, and the second forms the basis of the p216 gun.)
B35/S236 -- Cellular Automata -- D. Eppstein -- UCI Inf. & Comp. Sci.