Definition genGoodNarrow (n : nat) : nat -> G (option (Foo)) :=
 let
   fix aux_arb size (n : nat) : G (option (Foo)) :=
     match size with
     | O => backtrack [(1, ret (Some Foo1))]
     | S size' =>
         backtrack [ (1, ret (Some Foo1))
                   ; (1, bindGenOpt (aux_arb size' 0) (fun foo =>
                         match @decOpt (goodFooNarrow 1 foo) _  42 with
                         | Some true => ret (Some foo)
                         | _ => ret None
                         end
                     ))]
     end in fun sz => aux_arb sz n.

Lemma genGoodNarrow_equality n : genGoodNarrow n = @arbitrarySizeST _ (fun foo => goodFooNarrow n foo) _.