∀P,Q.arrows2 OBP P Q → Orelation_pair_setoid P Q ≝ λP,Q,c.c.
coercion Orelation_pair_setoid_of_arrows2_OBP.
+notation > "B ⇒_\obp2 C" right associative with precedence 72 for @{'arrows2_OBP $B $C}.
+notation "B ⇒\sub (\obp 2) C" right associative with precedence 72 for @{'arrows2_OBP $B $C}.
+interpretation "'arrows2_OBP" 'arrows2_OBP A B = (arrows2 OBP A B).
+
(*
definition BPext: ∀o: BP. form o ⇒ Ω \sup (concr o).
intros; constructor 1;