How do I use the cerberus set data type?

[val.m...@symbiont.io]

The subject says it all. I couldn't find any documentation for the sets, so this is what I tried and didn't work:

{
'field': {
'type': 'set',
'schema': {
'type':'string',
'allowed':['aa', 'bb', 'cc']
}
}
}

Your help would be appreciated.

Thank you!
Val