  We consider a stochastic process that describes several particles interacting
by either merging or annihilation. When two particles merge, they combine their
masses; when annihilation occurs, only the particle of smallest mass survives.
Particles start at the bottom of a binary tree of depth N and move towards the
root. Assuming that merging or annihilation happens independently at random, we
determine the limit law of the final mass of the system in the large N limit.
