$<\mathbb{B}$,$\vee_{b}>$ $\,\equiv$$_{\mbox{\scriptsize def}}$$\;\;$$<\mathbb{B}$, $\lambda$$x$,$y$. $x$ =b $y$, $\lambda$$x$,$y$. tt, $\lambda$$x$,$y$. $x$ $\vee_{b}$$y$, ff, $\lambda$$x$.$x$$>$