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