a)$-axiom. Let ${\ensuremath{\mbox{\boldmath $\mathcal{M}$}}\xspace}= (U,M)$ be the set assignment differing from ${\ensuremath{\mbox{\boldmath $\mathcal{M}$}}\xspace}_{\alpha}$ only on the interpretation of the choice symbol ${\mathtt{c}}$, for which we have ${{\mathtt{c}}^{\scriptscriptstyle M}} = {c^{\hbox{\tiny{+}}}}_{\