Prevent monsters to hit other monsters

function onStatsChange(cid, attacker, type, combat, value)
    -- This should block all damage monster cause on eachother expect player summons
    if isMonster(cid) and isMonster(attacker) then
        local master = getCreatureMaster(cid)
        if not master or not isPlayer(master) then
            return false
        end
    end

    return true
end