I experimentally found that:

1. It's base=none
2. Nothing works without fields starting with a dot. .items is a must.
3. Delimiter is ";"

I have experimentally determined that:

1. The base parameter must be passed a value of none to create a filter without a base.

2. Fields in the default wrapper object can be filtered; you must include .items in your filter to receive any data from inside it. (The base filter does; this is why filters with a base don't have this problem.)

3. This is technically a vector parameter; so, as with all vectors in the API, the delimiter is ; (U+0003B SEMICOLON).