Relation _HOLLOW

A hollow is an external hole, such as a depression. It is a region connected to the complement of its host.
Arity: 2
Domain: Region
Subrelation-Of: _hole
Axioms:
(=> (_Hollow ?A ?B)
    (Exists (?Y ?Z)
            (And (Material-Object ?Z)
                 (Host-Of ?Z ?A)
                 (Exists (?C)
                         (And (T-Complement ?Z ?C) (Identity ?Y ?C)))
                 (Connected ?A ?Y))))