fact: Definition empty_gen {A} : G (option A) := MkGen (fun _ _ => None).
imports: String micromega.Lia List Tactics TacticsUtil Instances Classes DependentClasses Sets Producer Generators EnumProofs Checker Decidability CheckerProofs mathcomp(ssreflect ssrfun ssrbool ssrnat eqtype seq) Ltac2(Ltac2)