markEvent = function markEvent(e) {

        // Fail if event is already marked
        if (e[EVENT_MARKER])
            return false;

        // Mark event otherwise
        e[EVENT_MARKER] = true;
        return true;

    }
Attempts to mark the given Event as having been handled by this
Guacamole.Keyboard. If the Event has already been marked as handled,
false is returned.

@param {Event} e
    The Event to mark.

@returns {Boolean}
    true if the given Event was successfully marked, false if the given
    Event was already marked.