pattern, common-lisp:structure-object, sb-pcl::slot-object, common-lisp:t

None

This pattern references another part of the pattern graph.

ref is the only pattern to introduce shared structure and circularity into the pattern graph, by referring to elements defined elsewhere.

(ref patterns in the full Relax NG syntax can be used to refer to any pattern definition in the grammar. Simplification normalizes the schema so that ref patterns only refer to definitions which have an element as their child.)