Definition not_regular_element {H : complete_heyting_algebra} (x : regular_element H) : regular_element H. Proof. use make_regular_element. - exact (¬ x). - abstract (use make_is_regular_element ; rewrite cha_not_not_not ; apply cha_le_refl). Defined.
Require Import UniMath.MoreFoundations.All. Require Import UniMath.Algebra.BinaryOperations. Require Import UniMath.OrderTheory.Lattice.Lattice. Require Import UniMath.OrderTheory.Lattice.Bounded. Require Import UniMath.OrderTheory.Lattice.Heyting. Require Import UniMath.OrderTheory.Lattice.CompleteHeyting. Require Import UniMath.OrderTheory.Lattice.DerivedLawsCompleteHeyting.