X-Git-Url: http://matita.cs.unibo.it/gitweb/?a=blobdiff_plain;f=matita%2Fmatita%2Flib%2Fbasics%2Frelations.ma;h=f469ddca29f72fe1eeaeea9d50752fefed34aad2;hb=a04bfe6d381b281db15e8b432f6f221576aad439;hp=d74380de43938650d6de559ba8e3e67384251f70;hpb=6906417a548f47a888fa5a43470dc3c5d7e4442b;p=helm.git

diff --git a/matita/matita/lib/basics/relations.ma b/matita/matita/lib/basics/relations.ma
index d74380de4..f469ddca2 100644
--- a/matita/matita/lib/basics/relations.ma
+++ b/matita/matita/lib/basics/relations.ma
@@ -167,6 +167,9 @@ definition bi_relation: Type[0] → Type[0] → Type[0]
 definition bi_reflexive: ∀A,B. ∀R:bi_relation A B. Prop
 ≝ λA,B,R. ∀x,y. R x y x y.
 
+definition bi_symmetric: ∀A,B. ∀R: bi_relation A B. Prop ≝ λA,B,R.
+                         ∀a1,a2,b1,b2. R a2 b2 a1 b1 → R a1 b1 a2 b2.
+
 definition bi_transitive: ∀A,B. ∀R: bi_relation A B. Prop ≝ λA,B,R.
                           ∀a1,a,b1,b. R a1 b1 a b →
                           ∀a2,b2. R a b a2 b2 → R a1 b1 a2 b2.