\begin{tabbing}
es{-}height(${\it es}$;$e$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=if\= first($e$)\+\+
\\[0ex]then if isrcv($e$) then es{-}height(${\it es}$;sender($e$))+1 else 0 fi 
\-\\[0ex]if\= isrcv($e$)\+
\\[0ex]then \=if es{-}height(${\it es}$;sender($e$)) $<$z es{-}height(${\it es}$;pred($e$))\+
\\[0ex]then es{-}height(${\it es}$;pred($e$))
\\[0ex]else es{-}height(${\it es}$;sender($e$))
\\[0ex]fi +1
\-\-\\[0ex]else es{-}height(${\it es}$;pred($e$))+1
\\[0ex]fi 
\\[0ex]
\-\\[0ex]{\em clarification:}
\\[0ex]
\\[0ex]es{-}height(${\it es}$;$e$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=if\= es{-}first(${\it es}$; $e$)\+\+
\\[0ex]then if es{-}isrcv(${\it es}$; $e$) then es{-}height(${\it es}$;es{-}sender(${\it es}$; $e$))+1 else 0 fi 
\-\\[0ex]if\= es{-}isrcv(${\it es}$; $e$)\+
\\[0ex]then \=if es{-}height(${\it es}$;es{-}sender(${\it es}$; $e$)) $<$z es{-}height(${\it es}$;es{-}pred(${\it es}$; $e$))\+
\\[0ex]then es{-}height(${\it es}$;es{-}pred(${\it es}$; $e$))
\\[0ex]else es{-}height(${\it es}$;es{-}sender(${\it es}$; $e$))
\\[0ex]fi +1
\-\-\\[0ex]else es{-}height(${\it es}$;es{-}pred(${\it es}$; $e$))+1
\\[0ex]fi 
\-\\[0ex]\emph{(recursive)}
\end{tabbing}