The Dual command returns the dual of the Boolean expression b, that is, the expression generated by replacing &and with &or, &or with &and, leaving &not fixed, and extending to the remaining Boolean operators by their formulas in terms of &and, &or, and &not.

with(Logic):

Dual(a &and (&not a) = false);

$a\vee \left(\neg a\right)=\mathrm{true}$

Dual(a &implies b);

$\neg \left(b\Rightarrow a\right)$

Dual((&not a) &nor b &iff c);

$\left(\left(\neg a\right)\uparrow b\right)⊻c$