On second thought don't stack HD when merging. Merge monster flags when merging. Merging costs the merged slime a turn.
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merge_to->hit_dice += initial_slime->hit_dice;
// Merge monster flags (mostly so that MF_CREATUED_NEUTRAL etc. are
// passed on if the merged slime subsequently splits. Hopefully
// this won't do anything weird.
merge_to->flags |= initial_slime->flags;
// Merging costs the combined slime some energy.
monsterentry* entry = get_monster_data(merge_to->type);
merge_to->speed_increment -= entry->energy_usage.move;
// This is dumb. With that said, the idea is that if 2 slimes merge
// you can gain a space by moving away the turn after (maybe this
// is too nice but there will probably be a lot of complaints about
// the damage on higher level slimes). So we subtracted some energy
// above but if merge_to hasn't moved yet this turn that will just
// cancel its turn in this round of world_reacts. So we are going
// to see if merge_to has gone already by checking its mindex
// (this works because handle_monsters just iterates over env.mons
// in ascending order)
if(initial_slime->mindex() < merge_to->mindex())
merge_to->speed_increment -= entry->energy_usage.move;