var z_see_img_actu = 0;
function z_see_img(an_img) {
    if (z_see_img_actu==an_img) return;
    $('img_'+z_see_img_actu).setStyles({'opacity':'0','visibility':'hidden'});
    $('img_'+an_img).setStyles({'opacity':'0','visibility':'hidden'});
    $('img_'+an_img).setStyles({'opacity':'1','visibility':'visible'});
    z_see_img_actu = an_img;
}

function add(nom, max) {
    if($(nom).value < max){
        $(nom).value ++;
    }
    else{
        alert('Maximum atteint');
    }
} 

function substract(nom) {
    if($(nom).value > 0){
        $(nom).value --;
    }
}