let $w$,$x$,$y$,$z$ = $a$ in $t$($w$;$x$;$y$;$z$)
\\[0ex]$\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$let $w$,${\it zz}_{1}$ = $a$ in let $x$,${\it zz}_{2}$ = ${\it zz}_{1}$ in let $y$,$z$ = ${\it zz}_{2}$ in $t$($w$;$x$;$y$;$z$)