// Monsters that are smart enough to use stairs can push past monsters
    // that are too stupid to use stairs (e.g. zombies).
    if (m1->can_use_stairs() && !m2->can_use_stairs())

    // If they're the same holiness, monsters smart enough to use stairs can
    // push past monsters too stupid to use stairs (so that e.g. non-zombified
    // undead can push past zombified undead).
    if (m1->holiness() == m2->holiness() && m1->can_use_stairs()
        && !m2->can_use_stairs())
    {

    }