\begin{figure}[H]%
\centering
\subcaptionbox{transferableOperation = nil.}{\fbox{\includegraphics[keepaspectratio,width=0.45\textwidth]{./images/transformational_semantics_of_oz/transferableOperation_null.png}}}%
\hspace{\fill}
\hspace{1em}%
\subcaptionbox{transferableOperation = talk.}{\fbox{\includegraphics[keepaspectratio,width=0.45\textwidth]{./images/transformational_semantics_of_oz/transferableOperation.png}}}%
\caption{Mapping transferable operation's variable.}
\label{tra_ref}%
\end{figure}
\lstinputlisting[backgroundcolor=\color{white},caption={ Nullref and Ref processes in ABC code.},captionpos=b, label={tra_ref_listing}]{listings/ref_OZ.abc}