\begin{document}

\ontology#testOntology
	\struct#test[root=true]
		\field
			\childRef[ref=a]
	\struct#a
		\field
			\childRef[ref=b]
	\struct#b
		\primitive[type=string, optional=true]

\begin{test}
\a{!}
\b
\a
\end{test}
\end{document}