function toggleBackButtonsVisibility (){
    if (history.length > 0){
        toggleBackButtonVisibility("inline");
    }else{
        toggleBackButtonVisibility("none");
    }
}

function toggleBackButtonVisibility (visibilty){
    backButtons = document.getElementsByName("btnTerug");
    for(var i=0;i<backButtons.length;i++){
        backButtons[i].style.display = visibilty;
    }
}