Someone

vesta.xul has:

<commandset id="commonCommands"/>
<?xul-overlay href="chrome://vesta/content/common.xul"?>

common.xul contains:
<overlay id="vestaCommonOverlay" xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul">
    <commandset id="commonCommands">
        <command id="cmdLogin"
            oncommand="Vesta.doLogin(document.getElementById('loginEmail').value,
                       document.getElementById('loginPass').value);"/>
        <command id="cmdQuit" oncommand="Vesta.quit(false);"/>
        <command id="cmdAbout" oncommand="Vesta.about();"/>
    </commandset>
</overlay>