Inductive quantity : Type := Q32 | Q64.Inductive quantity : Type := Q32 | Q64.Inductive quantity : Type := Q32 | Q64.Definition quantity_eq (q1 q2: quantity) : {q1 = q2} + {q1 <> q2}.Definition size_quantity_nat (q: quantity) :=
  match q with Q32 => 4%nat | Q64 => 8%nat end.

Lemma size_quantity_nat_pos: forall q, exists n, size_quantity_nat q = S n.