reducible($a$) $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$\exists$$b$, $c$:$\mathbb{Z}^{-\circ}$. $\neg$($b$ $\sim$ 1) \& $\neg$($c$ $\sim$ 1) \& $a$ $=$ $b$$\cdot$$c$