I just realized that the options "type" and "options" in CollectionType are rather ambiguous. Maybe we can add the aliases "entry_type" and "entry_options" to make them more readable:

$builder->add('synonyms', 'collection', array(
    'entry_type' => 'synonym',
    'entry_options' => array(...),
));

Especially in big forms, it's hard to understand the exact form structure with the current naming.