proposition: Lemma hzsign_hzsign (i:hz) : - - i = i.
proof: Proof. apply (grinvinv ℤ). Defined.
imports: UniMath.Foundations.NaturalNumbers UniMath.MoreFoundations.PartA UniMath.MoreFoundations.NegativePropositions UniMath.Algebra.Monoids UniMath.Algebra.Groups UniMath.Algebra.RigsAndRings UniMath.NumberSystems.NaturalNumbersAlgebra