\begin{tabbing}
mux{-}component(${\it Ca}$;${\it Cb}$)($X$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=let ${\it Ra}$,${\it Ya}$ = (${\it Ca}$(interface{-}left($X$)))\+
\\[0ex]in
\\[0ex]let ${\it Rb}$,${\it Yb}$ = (${\it Cb}$(interface{-}right($X$))) in $<$scheme{-}plus(${\it Ra}$;${\it Rb}$), interface{-}union(${\it Ya}$;${\it Yb}$)$>$
\-
\end{tabbing}