--- /dev/null
+(**************************************************************************)
+(* ___ *)
+(* ||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.