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(* as published by the Free Software Foundation; either version 2.1   *)
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(*                                                                    *)
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(* GNU General Public License for more details.                       *)
(*                                                                    *)
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(* 02110-1301 USA                                                     *)
Set Asymmetric Patterns.
Unset Standard Proposition Elimination Names.
Require Export ZArith.
Require Export Omega.
(**********************************************************************  CONFIGURATION  **********************************************************************)
Section CONFIGURATION.
Definition counter := Z.
Record Config : Set := mkconfig {co : counter}.
Variable x : Z.
Definition mk_init_config := mkconfig x.
End CONFIGURATION.
(**********************************************************************  TRANSITIONS  **********************************************************************)
Section TRANSITIONS.
Definition decrement_trans (c : Config) := mkconfig (co c - 1)%Z.
Definition divide_trans (c : Config) := mkconfig (Zeven.Zdiv2 (co c)).
Inductive transition (c : Config) : Set :=
| decrement : (co c > 0)%Z -> Zeven.Zodd (co c) -> transition c
| divide : (co c > 0)%Z -> Zeven.Zeven (co c) -> transition c.
Definition next_config (c : Config) (t : transition c) :=
match t with
| decrement h1 h2 => decrement_trans c
| divide h1 h2 => divide_trans c
end.
Variable x : Z.
Inductive legal : Config -> Prop :=
| begin : legal (mk_init_config x)
| after :
forall (c : Config) (t : transition c),
legal c -> legal (next_config c t).
End TRANSITIONS.
(* positive *)

Lemma counter_remains_positive_or_null :
forall (x : Z) (c : Config), (x >= 0)%Z -> legal x c -> (co c >= 0)%Z.