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diff --git a/matita/matita/contribs/lambdadelta/ground_2/lib/stream_hdtl.ma b/matita/matita/contribs/lambdadelta/ground_2/lib/stream_hdtl.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                  *)
+(*                                                                        *)
+(**************************************************************************)
+
+include "ground_2/notation/functions/downspoon_2.ma".
+include "ground_2/lib/stream_eq.ma".
+include "ground_2/lib/arith.ma".
+
+(* STREAMS ******************************************************************)
+
+definition hd (A:Type[0]): stream A → A ≝
+              λt. match t with [ seq a _ ⇒ a ].
+
+definition tl (A:Type[0]): stream A → stream A ≝
+              λt. match t with [ seq _ t ⇒ t ].
+
+interpretation "tail (stream)" 'DownSpoon A t = (tl A t).
+
+(* basic properties *********************************************************)
+
+lemma hd_rew (A) (a) (t): a = hd A (a⨮t).
+// qed.
+
+lemma tl_rew (A) (a) (t): t = tl A (a⨮t).
+// qed.
+
+lemma eq_stream_split (A) (t): (hd … t) ⨮ ⫰t ≗{A} t.
+#A * //
+qed.