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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 "lift.ma".
+
+(* LENGTH *******************************************************************)
+
+(* Note: this gives the number of abstractions and applications in M *)
+let rec length M on M ≝ match M with
+[ VRef i   ⇒ 0
+| Abst A   ⇒ length A + 1
+| Appl B A ⇒ (length B) + (length A) + 1
+].
+
+interpretation "term length"
+   'card M = (length M).
+
+lemma length_lift: ∀h,M,d. |↑[d, h] M| = |M|.
+#h #M elim M -M normalize //
+qed.