sub _addEventListener {
    my ($self,$browser,$events) = @_;
    $events ||= $self->events;
    $events = [$events]
        unless ref $events;

    # This registers multiple events for a one-shot event
    my $make_semaphore = $self->repl->declare(<<'JS');
function(browser,events) {
    var lock = {};
    lock.busy = 0;
    var b = browser;
    var listeners = [];
    for( var i = 0; i < events.length; i++) {
        var evname = events[i];
        var callback = (function(listeners,evname){
            return function(e) {
                if (! lock.busy) {
                    lock.busy++;
                    lock.event = evname;
                    lock.js_event = {};
                    lock.js_event.target = e.originalTarget;
                    lock.js_event.type = e.type;
                    alert("Caught first event " + e.type + " " + e.message);
                } else {
                    alert("Caught duplicate event " + e.type + " " + e.message);
                };
                for( var j = 0; j < listeners.length; j++) {
                    b.removeEventListener(listeners[j][0],listeners[j][1],true);
                };
            };
        })(listeners,evname);
        listeners.push([evname,callback]);
        b.addEventListener(evname,callback,true);
    };
    return lock
}
JS
    return $make_semaphore->($browser,$events);
};