closed intersection collection type is a collection of collection types which all share the following property: Whenever COLLTYPE is an instance of this collection, and COLL1 and COLL2 are instances of COLLTYPE, then the intersection of COLL1 and COLL2 is also an instance of COLLTYPE. For example, configuration is an instance of this collection. somewhat round thing and cube are both instances of configuration, so (Collection Intersection Fn (The Set somewhat round thing cube)), the collection of somewhat round cubes, is also an instance of configuration.

Note that it is possible that this intersection may be empty. square and circle are both instances of configuration, so is (Collection Intersection Fn (The Set square circle)), even though this collection is necessarily empty.