\begin{document}

\domain#testDomain
	\struct#test[isRoot=true]
		\field
			\childRef[ref=a]
			\childRef[ref=b]
	\struct#a
		\field
			\childRef[ref=b]
	\struct#b[transparent=true]
		\primitive[type=string,optional=true]

\begin{test}
\a
\include{./include_child.osml}
\b
Test
\end{test}
\end{document}