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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                  *)
+(*                                                                        *)
+(**************************************************************************)
+
+
+
+include "attic/rings.ma".
+
+record is_field (R:ring) (inv:∀x:R.x ≠ 0 → R) : Prop
+≝
+ {  (* multiplicative abelian properties *)
+    mult_comm_: symmetric ? (mult R);
+    (* multiplicative group properties *)
+    inv_inverse_: ∀x.∀p: x ≠ 0. inv x p * x = 1
+ }.
+
+lemma opp_opp: ∀R:ring. ∀x:R. --x=x.
+intros; 
+apply (cancellationlaw ? (-x) ? ?); 
+rewrite > (opp_inverse R x); 
+rewrite > plus_comm;
+rewrite > opp_inverse;
+reflexivity.
+qed.
+
+let rec sum (C:Type) (plus:C→C→C) (zero,one:C) (n:nat) on n  ≝
+ match n with
+  [ O ⇒ zero
+  | (S m) ⇒ plus one (sum C plus zero one m)
+  ].
+
+record field : Type \def
+ { f_ring:> ring;
+   inv: ∀x:f_ring. x ≠ 0 → f_ring;
+   field_properties: is_field f_ring inv
+ }.
+ 
+theorem mult_comm: ∀F:field.symmetric ? (mult F).
+ intro;
+ apply (mult_comm_ ? ? (field_properties F)).
+qed.
+
+theorem inv_inverse: ∀F:field.∀x:F.∀p: x ≠ 0. (inv ? x p)*x = 1.
+ intro;
+ apply (inv_inverse_ ? ? (field_properties F)).
+qed.
+
+(*CSC: qua funzionava anche mettendo ? al posto della prima F*)
+definition sum_field ≝
+ λF:field. sum F (plus F) 0 1.