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