function release_key(keysym) {

        // Only release if pressed
        if (guac_keyboard.pressed[keysym]) {
            
            // Mark key as released
            delete guac_keyboard.pressed[keysym];

            // Stop repeat
            window.clearTimeout(key_repeat_timeout);
            window.clearInterval(key_repeat_interval);

            // Send key event
            if (keysym !== null && guac_keyboard.onkeyup)
                guac_keyboard.onkeyup(keysym);

        }

    }
Marks a key as released, firing the keyup event if registered.

@private
@param keysym The keysym of the key to release.