construct the dual of a Boolean expression
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 ¬ fixed, and extending to the remaining Boolean operators by their formulas in terms of &and, &or, and ¬.
Dual⁡a &and `¬`⁡a=false
Dual⁡a &implies b
Dual⁡`¬`⁡a &nor b &iff c
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