A config key representing a set of values.
If a value is set using this *typed* key, it is _added_ to the set
(with a warning issued if a collection is passed in).
If a value is set against an equivalent *untyped* key which *is* a collection,
it will be treated as a set upon discovery and used as a base to which subkey values are added.
If a value is discovered against this key which is not a map or collection,
it is ignored.
To add all items in a collection, to add a collection as a single element,
to clear the list, or to set a collection (clearing first),
use the relevant
Specific values can be added in a replaceable way by referring to a subkey.