eapply (H n); try eassumption. ssromega.
  Qed.

  Instance sizedSizeMonotonic
           A (gen : nat -> G Apply H0; eassumption.
  Qed.

  Instance sizedSizeMonotonicOpt
          A (gen : nat -> G (optiProof. by case: g => g; rewrite /semGenSize. Qed.
  
  Instance unsizedResize

Lemma semGenSizeInhabited {A}