frequency(${\it eq}$;$f$;$x$;$p$;$q$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\forall$$m$:$\mathbb{N}$, $k$:$\mathbb{N}$. $\exists$$j$:$\mathbb{N}$. (($k$ $<$ $j$) c$\wedge$ $\mid$\#\{i$<$$j$$\mid$$f$ i ${\it eq}$ $x$\}/$j$ {-} $p$/$q$$\mid$ $<$ 1/$m$)