\begin{tabbing}
same\_order($x_{1}$;$y_{1}$;$x_{2}$;$y_{2}$;$L$;$T$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$\=strong\_before($x_{1}$; $y_{1}$; $L$; $T$)\+
\\[0ex]$\Rightarrow$ ($x_{2}$ $\in$ $L$ $\in$ $T$)
\\[0ex]$\Rightarrow$ ($y_{2}$ $\in$ $L$ $\in$ $T$)
\\[0ex]$\Rightarrow$ strong\_before($x_{2}$; $y_{2}$; $L$; $T$)
\-
\end{tabbing}