var stack = [];
var lastState = document.getElementById('drop_zone_content').innerHTML;
stack.push(lastState);
var observer = new MutationObserver(function(div) {

if(current_hash != old_hash){

var newState = document.getElementById('drop_zone_content').innerHTML;
stack.push(newState);
old_hash = current_hash;

}
});

function undo(){
	document.getElementById('drop_zone_content').innerHTML = stack.pop();
}