fact: Definition Only_even only_even n := if eq_nat_dec n 0 then 1 else if eq_nat_dec n 1 then 1 + only_even 1 else only_even (n - 2).
imports: TLC(LibTactics LibLogic LibReflect LibFun LibEpsilon LibList LibInt LibNat LibProd LibSum LibRelation LibWf LibFix LibStream) ZifyNat