--- /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/semigroups".
+
+include "higher_order_defs/functions.ma".
+
+(* Magmas *)
+
+record Magma : Type≝
+ { carrier:> Type;
+ op: carrier → carrier → carrier
+ }.
+
+notation "hvbox(a break \middot b)"
+ left associative with precedence 55
+for @{ 'magma_op $a $b }.
+
+interpretation "magma operation" 'magma_op a b =
+ (cic:/matita/algebra/semigroups/op.con _ a b).
+
+(* Semigroups *)
+
+record isSemiGroup (M:Magma) : Prop≝
+ { op_associative: associative ? (op M) }.
+
+record SemiGroup : Type≝
+ { magma:> Magma;
+ semigroup_properties:> isSemiGroup magma
+ }.
+
+definition is_left_unit ≝
+ λS:SemiGroup. λe:S. ∀x:S. e·x = x.
+
+definition is_right_unit ≝
+ λS:SemiGroup. λe:S. ∀x:S. x·e = x.
+
+theorem is_left_unit_to_is_right_unit_to_eq:
+ ∀S:SemiGroup. ∀e,e':S.
+ is_left_unit ? e → is_right_unit ? e' → e=e'.
+ intros;
+ rewrite < (H e');
+ rewrite < (H1 e) in \vdash (? ? % ?).
+ reflexivity.
+qed.