\begin{tabbing}
$A$ $\parallel$ $B$
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=i\=f Rplus?($A$)$\rightarrow$ Rplus{-}left($A$) $\parallel$ $B$ \& Rplus{-}right($A$) $\parallel$ $B$\+\+
\\[0ex]; Rplus?($B$)$\rightarrow$ $A$ $\parallel$ Rplus{-}left($B$) \& $A$ $\parallel$ Rplus{-}right($B$)
\\[0ex]; Rnone?($A$)$\rightarrow$ True
\\[0ex]; Rnone?($B$)$\rightarrow$ True
\\[0ex]; \=R{-}loc($A$) = R{-}loc($B$)$\rightarrow$\+
\\[0ex]Rds($A$) $\parallel$ Rds($B$) \& Rda($A$) $\parallel$ Rda($B$)
\\[0ex]\& \=if R{-}base{-}domain($A$) = R{-}base{-}domain($B$)$\rightarrow$ $A$ $=$ $B$\+
\\[0ex]else R{-}frame{-}compat($A$;$B$) \& R{-}frame{-}compat($B$;$A$) fi
\-\-\-\\[0ex]else R{-}interface{-}compat($A$;$B$) \& R{-}interface{-}compat($B$;$A$) fi
\-\\[0ex]\emph{(recursive)}
\end{tabbing}