itree_add_bubble
static struct bubble *itree_add_bubble(struct event *eventp,
frombp = itree_add_bubble(frome, B_FROM, 0, 0);
tobp = itree_add_bubble(toe, B_TO, 0,
G.frombp = itree_add_bubble(G.frome, B_FROM, G.n, 0);
G.tobp = itree_add_bubble(G.toe, B_TO, G.k, G.generation);