Different "mix-in" classes are provided that implement the relationship protocol.
Summary: This class is mixed into sheet classes that have a parent.
Summary: This class is mixed into sheet classes that will never have children.
Summary: This class is mixed into sheet classes that have at most a single child.
Summary: This class is mixed into sheet classes that may have zero or more children.
Common Lisp Interface Manager 2.0 User's Guide - 7 Aug 2017