Definition unop_single (sem: float32 -> float32) (x: aval) :=
  match x with FS n => FS (sem n) | _ => ntop1 x end.

Lemma unop_single_sound: forall sem v x, vmatch v x -> vmatch (match v with Vsingle i => Vsingle(sem i) | _ => Vundef end) (unop_single sem x).