\begin{tabbing}
w{-}match($w$; $l$; $t$; ${\it t'}$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=$\parallel$w{-}snds($w$; $l$; $t$)$\parallel\leq_{2}\parallel$w{-}rcvs($w$; $l$; ${\it t'}$)$\parallel$\+
\\[0ex]$\wedge_{2}$ $\parallel$w{-}rcvs($w$; $l$; ${\it t'}$)$\parallel<_{2}\parallel$w{-}snds($w$; $l$; $t$)$\parallel$+$\parallel$onlnk($l$;w{-}m($w$; source($l$); $t$))$\parallel$
\-
\end{tabbing}