\begin{tabbing}
w{-}d($w$;$e$;${\it e'}$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=if w{-}time($w$; $e$)$\leq_{2}$w{-}time($w$; ${\it e'}$)$\rightarrow$ w{-}time($w$; ${\it e'}$)$-$w{-}time($w$; $e$)\+
\\[0ex]else w{-}time($w$; $e$)$-$w{-}time($w$; ${\it e'}$) fi
\-
\end{tabbing}