Definition unary_constructor_sound (cstr: expr -> expr) (sem: val -> val) : Prop :=
  forall le a x,
  eval_expr ge sp e m le a x ->
  exists v, eval_expr ge sp e m le (cstr a) v /\ Val.lessdef (sem x) v.Definition binary_constructor_sound (cstr: expr -> expr -> expr) (sem: val -> val -> val) : Prop :=
  forall le a x b y,
  eval_expr ge sp e m le a x ->
  eval_expr ge sp e m le b y ->
  exists v, eval_expr ge sp e m le (cstr a b) v /\ Val.lessdef (sem x y) v.Definition partial_unary_constructor_sound (cstr: expr -> expr) (sem: val -> option val) : Prop :=
  forall le a x y,
  eval_expr ge sp e m le a x ->
  sem x = Some y ->
  exists v, eval_expr ge sp e m le (cstr a) v /\ Val.lessdef y v.Definition partial_binary_constructor_sound (cstr: expr -> expr -> expr) (sem: val -> val -> option val) : Prop :=
  forall le a x b y z,
  eval_expr ge sp e m le a x ->
  eval_expr ge sp e m le b y ->
  sem x y = Some z ->
  exists v, eval_expr ge sp e m le (cstr a b) v /\ Val.lessdef z v.

Theorem eval_negl: unary_constructor_sound negl Val.negl.