(** Let (X, A, a) and (Y, B, b) be two pointed graphs (with X,Y : Typ1).
    The unordered pair formed by the sets represented by these pointed
    graphs is itself represented by the pointed graph

      ((sum X Y), (PAIR X A a Y B b), (out X Y))

    whose edge relation (PAIR X A a Y B b) : (Rel (sum X Y)) is
    defined by the following four clauses :

    1. Delocate A in the new graph via (inl X Y):

         if (A x' x), then (PAIR X A a Y B b (inl X Y x') (inl X Y x))

    2. Delocate B in the new graph via (inr X Y):

         if (B y' y), then (PAIR X A a Y B b (inr X Y y') (inr X Y y))

    3. Connect the (image of the) root a to the new root (out X Y):

         (PAIR X A a Y B b (inl X Y a) (out X Y))

    4. Connect the (image of the) root b to the new root (out X Y):

         (PAIR X A a Y B b (inr X Y b) (out X Y))

   As usual, we define this relation by a direct impredicative encoding: *)

Definition PAIR (X : Typ1) (A : Rel X) (a : X) (Y : Typ1) 
  (B : Rel Y) (b : Y) (z' z : sum X Y) :=
  forall E : Prop,
  (forall x x' : X,
   eq (sum X Y) z (inl X Y x) -> eq (sum X Y) z' (inl X Y x') -> A x' x -> E) ->
  (forall y y' : Y,
   eq (sum X Y) z (inr X Y y) -> eq (sum X Y) z' (inr X Y y') -> B y' y -> E) ->
  (eq (sum X Y) z' (inl X Y a) -> eq (sum X Y) z (out X Y) -> E) ->
  (eq (sum X Y) z' (inr X Y b) -> eq (sum X Y) z (out X Y) -> E) -> E.

Lemma PAIR_in2 : forall (X : Typ1) (A : Rel X) (a : X) (Y : Typ1) (B : Rel Y) (b y y' : Y), B y' y -> PAIR X A a Y B b (inr X Y y') (inr X Y y).