case(kind($e$))
\\[0ex]act($a$) =$>$ $f$($a$)
\\[0ex]rcv($l$,${\it tg}$) =$>$ $g$($l$;${\it tg}$)
\\[0ex]
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$if isrcv($e$)$\rightarrow$ $g$(lnk($e$);tag($e$)) else $f$(act($e$)) fi