For\{$T$,${\it op}$,${\it id}$\} $x$ $\in$ ${\it as}$. $f$($x$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$reduce(${\it op}$;${\it id}$;map($\lambda$$x$:$T$. $f$($x$);${\it as}$))