function allowDrop(ev)
            {
                if (ev.target.getAttribute("droppable") == "false"){
                    ev.dataTransfer.dropEffect = "none"; // dropping is not allowed
                                      ev.preventDefault();
                }
                else{
                    ev.dataTransfer.dropEffect = "all"; // drop it like it's hot
                                      ev.preventDefault();
                }
            }