(** [sumbool] is a boolean type equipped with the justification of
    their value *)

Inductive sumbool (A B:Prop) : Set :=
  | left : A -> {A} + {B}
  | right : B -> {A} + {B}
 where "{ A } + { B }" := (sumbool A B) : type_scope.(** [sumbool] is a boolean type equipped with the justification of
    their value *)

Inductive sumbool (A B:Prop) : Set :=
  | left : A -> {A} + {B}
  | right : B -> {A} + {B}
 where "{ A } + { B }" := (sumbool A B) : type_scope.(** [sumbool] is a boolean type equipped with the justification of
    their value *)

Inductive sumbool (A B:Prop) : Set :=
  | left : A -> {A} + {B}
  | right : B -> {A} + {B}
 where "{ A } + { B }" := (sumbool A B) : type_scope.(** [sumbool] is a boolean type equipped with the justification of
    their value *)

Inductive sumbool (A B:Prop) : Set :=
  | left : A -> {A} + {B}
  | right : B -> {A} + {B}
 where "{ A } + { B }" := (sumbool A B) : type_scope.(** [sumor] is an option type equipped with the justification of why
    it may not be a regular value *)

#[universes(template)]
Inductive sumor (A:Type) (B:Prop) : Type :=
  | inleft : A -> A + {B}
  | inright : B -> A + {B}
 where "A + { B }" := (sumor A B) : type_scope.(** [sumor] is an option type equipped with the justification of why
    it may not be a regular value *)

#[universes(template)]
Inductive sumor (A:Type) (B:Prop) : Type :=
  | inleft : A -> A + {B}
  | inright : B -> A + {B}
 where "A + { B }" := (sumor A B) : type_scope.(** [sumor] is an option type equipped with the justification of why
    it may not be a regular value *)

#[universes(template)]
Inductive sumor (A:Type) (B:Prop) : Type :=
  | inleft : A -> A + {B}
  | inright : B -> A + {B}
 where "A + { B }" := (sumor A B) : type_scope.(** [sumor] is an option type equipped with the justification of why
    it may not be a regular value *)

#[universes(template)]
Inductive sumor (A:Type) (B:Prop) : Type :=
  | inleft : A -> A + {B}
  | inright : B -> A + {B}
 where "A + { B }" := (sumor A B) : type_scope.

Lemma Choice2 : (forall x:S, {y:S' & R' x y}) -> {f:S -> S' & forall z:S, R' z (f z)}.