option('driver', 
    type: 'combo', 
    choices: ['epoll', 'cfrunloop', 'kqueue', 'winapi', 'select', 'libasync', 'generic', 'auto'], 
    value: 'auto',
    description: 'Which async backend driver to use',
    yield: true
)
option('use_gaia', type: 'feature', value: 'auto')