+(**************************************************************************)
+(* ___ *)
+(* ||M|| *)
+(* ||A|| A project by Andrea Asperti *)
+(* ||T|| *)
+(* ||I|| Developers: *)
+(* ||T|| The HELM team. *)
+(* ||A|| http://helm.cs.unibo.it *)
+(* \ / *)
+(* \ / This file is distributed under the terms of the *)
+(* v GNU General Public License Version 2 *)
+(* *)
+(**************************************************************************)
+
+include "ground_2/notation/relations/runion_3.ma".
+include "ground_2/relocation/rtmap_sle.ma".
+
+coinductive sor: relation3 rtmap rtmap rtmap ≝
+| sor_pp: ∀f1,f2,f,g1,g2,g. sor f1 f2 f → ↑f1 = g1 → ↑f2 = g2 → ↑f = g → sor g1 g2 g
+| sor_np: ∀f1,f2,f,g1,g2,g. sor f1 f2 f → ⫯f1 = g1 → ↑f2 = g2 → ⫯f = g → sor g1 g2 g
+| sor_pn: ∀f1,f2,f,g1,g2,g. sor f1 f2 f → ↑f1 = g1 → ⫯f2 = g2 → ⫯f = g → sor g1 g2 g
+| sor_nn: ∀f1,f2,f,g1,g2,g. sor f1 f2 f → ⫯f1 = g1 → ⫯f2 = g2 → ⫯f = g → sor g1 g2 g
+.
+
+interpretation "union (rtmap)"
+ 'RUnion f1 f2 f = (sor f1 f2 f).
+
+(* Basic inversion lemmas ***************************************************)
+
+lemma sor_inv_ppx: ∀g1,g2,g. g1 ⋓ g2 ≡ g → ∀f1,f2. ↑f1 = g1 → ↑f2 = g2 →
+ ∃∃f. f1 ⋓ f2 ≡ f & ↑f = g.
+#g1 #g2 #g * -g1 -g2 -g
+#f1 #f2 #f #g1 #g2 #g #Hf #H1 #H2 #H0 #x1 #x2 #Hx1 #Hx2 destruct
+try (>(injective_push … Hx1) -x1) try (>(injective_next … Hx1) -x1)
+try elim (discr_push_next … Hx1) try elim (discr_next_push … Hx1)
+try (>(injective_push … Hx2) -x2) try (>(injective_next … Hx2) -x2)
+try elim (discr_push_next … Hx2) try elim (discr_next_push … Hx2)
+/2 width=3 by ex2_intro/
+qed-.
+
+lemma sor_inv_npx: ∀g1,g2,g. g1 ⋓ g2 ≡ g → ∀f1,f2. ⫯f1 = g1 → ↑f2 = g2 →
+ ∃∃f. f1 ⋓ f2 ≡ f & ⫯f = g.
+#g1 #g2 #g * -g1 -g2 -g
+#f1 #f2 #f #g1 #g2 #g #Hf #H1 #H2 #H0 #x1 #x2 #Hx1 #Hx2 destruct
+try (>(injective_push … Hx1) -x1) try (>(injective_next … Hx1) -x1)
+try elim (discr_push_next … Hx1) try elim (discr_next_push … Hx1)
+try (>(injective_push … Hx2) -x2) try (>(injective_next … Hx2) -x2)
+try elim (discr_push_next … Hx2) try elim (discr_next_push … Hx2)
+/2 width=3 by ex2_intro/
+qed-.
+
+lemma sor_inv_pnx: ∀g1,g2,g. g1 ⋓ g2 ≡ g → ∀f1,f2. ↑f1 = g1 → ⫯f2 = g2 →
+ ∃∃f. f1 ⋓ f2 ≡ f & ⫯f = g.
+#g1 #g2 #g * -g1 -g2 -g
+#f1 #f2 #f #g1 #g2 #g #Hf #H1 #H2 #H0 #x1 #x2 #Hx1 #Hx2 destruct
+try (>(injective_push … Hx1) -x1) try (>(injective_next … Hx1) -x1)
+try elim (discr_push_next … Hx1) try elim (discr_next_push … Hx1)
+try (>(injective_push … Hx2) -x2) try (>(injective_next … Hx2) -x2)
+try elim (discr_push_next … Hx2) try elim (discr_next_push … Hx2)
+/2 width=3 by ex2_intro/
+qed-.
+
+lemma sor_inv_nnx: ∀g1,g2,g. g1 ⋓ g2 ≡ g → ∀f1,f2. ⫯f1 = g1 → ⫯f2 = g2 →
+ ∃∃f. f1 ⋓ f2 ≡ f & ⫯f = g.
+#g1 #g2 #g * -g1 -g2 -g
+#f1 #f2 #f #g1 #g2 #g #Hf #H1 #H2 #H0 #x1 #x2 #Hx1 #Hx2 destruct
+try (>(injective_push … Hx1) -x1) try (>(injective_next … Hx1) -x1)
+try elim (discr_push_next … Hx1) try elim (discr_next_push … Hx1)
+try (>(injective_push … Hx2) -x2) try (>(injective_next … Hx2) -x2)
+try elim (discr_push_next … Hx2) try elim (discr_next_push … Hx2)
+/2 width=3 by ex2_intro/
+qed-.
+
+(* Basic properties *********************************************************)
+
+let corec sor_refl: ∀f. f ⋓ f ≡ f ≝ ?.
+#f cases (pn_split f) * #g #H
+[ @(sor_pp … H H H) | @(sor_nn … H H H) ] -H //
+qed.
+
+let corec sor_sym: ∀f1,f2,f. f1 ⋓ f2 ≡ f → f2 ⋓ f1 ≡ f ≝ ?.
+#f1 #f2 #f * -f1 -f2 -f
+#f1 #f2 #f #g1 #g2 #g #Hf * * * -g1 -g2 -g
+[ @sor_pp | @sor_pn | @sor_np | @sor_nn ] /2 width=7 by/
+qed-.