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diff --git a/matita/library/algebra/monoids.ma b/matita/library/algebra/monoids.ma
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+(**************************************************************************)
+(*       ___                                                              *)
+(*      ||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                  *)
+(*                                                                        *)
+(**************************************************************************)
+
+set "baseuri" "cic:/matita/algebra/monoids/".
+
+include "algebra/semigroups.ma".
+
+record PreMonoid : Type ≝
+ { magma:> Magma;
+   e: magma
+ }.
+
+record isMonoid (M:PreMonoid) : Prop ≝
+ { is_semi_group:> isSemiGroup M;
+   e_is_left_unit:
+    is_left_unit (mk_SemiGroup ? is_semi_group) (e M);
+   e_is_right_unit:
+    is_right_unit (mk_SemiGroup ? is_semi_group) (e M)
+ }.
+ 
+record Monoid : Type ≝
+ { premonoid:> PreMonoid;
+   monoid_properties:> isMonoid premonoid 
+ }.
+
+notation "1" with precedence 89
+for @{ 'munit }.
+
+interpretation "Monoid unit" 'munit =
+ (cic:/matita/algebra/monoids/e.con _).
+  
+definition is_left_inverse ≝
+ λM:Monoid.
+  λopp: M → M.
+   ∀x:M. (opp x)·x = 1.
+   
+definition is_right_inverse ≝
+ λM:Monoid.
+  λopp: M → M.
+   ∀x:M. x·(opp x) = 1.
+
+theorem is_left_inverse_to_is_right_inverse_to_eq:
+ ∀M:Monoid. ∀l,r.
+  is_left_inverse M l → is_right_inverse M r → 
+   ∀x:M. l x = r x.
+ intros;
+ generalize in match (H x); intro;
+ generalize in match (eq_f ? ? (λy.y·(r x)) ? ? H2);
+ simplify; fold simplify (op M);
+ intro; clear H2;
+ generalize in match (op_associative ? (is_semi_group ? (monoid_properties M)));
+ intro;
+ rewrite > H2 in H3; clear H2;
+ rewrite > H1 in H3;
+ rewrite > (e_is_left_unit ? (monoid_properties M)) in H3;
+ rewrite > (e_is_right_unit ? (monoid_properties M)) in H3;
+ assumption.
+qed.