Inductive D_queue : queue Message -> Prop :=
  | D_empty : D_queue (empty Message)
  | D_dec : forall qm : queue Message, D_queue (input Message dec qm).Inductive D_queue : queue Message -> Prop :=
  | D_empty : D_queue (empty Message)
  | D_dec : forall qm : queue Message, D_queue (input Message dec qm).Inductive D_queue : queue Message -> Prop :=
  | D_empty : D_queue (empty Message)
  | D_dec : forall qm : queue Message, D_queue (input Message dec qm).Inductive alternate : queue Message -> Prop :=
  | alt_null : alternate (empty Message)
  | alt_single_inc :
      forall s0 : Site,
      alternate (input Message (inc_dec s0) (empty Message))
  | alt_any_alt :
      forall (qm : queue Message) (m : Message),
      (forall s : Site, m <> inc_dec s) ->
      alternate qm -> alternate (input Message m qm)
  | alt_inc_dec :
      forall (qm : queue Message) (s0 : Site),
      alternate qm ->
      alternate (input Message (inc_dec s0) (input Message dec qm)).Inductive alternate : queue Message -> Prop :=
  | alt_null : alternate (empty Message)
  | alt_single_inc :
      forall s0 : Site,
      alternate (input Message (inc_dec s0) (empty Message))
  | alt_any_alt :
      forall (qm : queue Message) (m : Message),
      (forall s : Site, m <> inc_dec s) ->
      alternate qm -> alternate (input Message m qm)
  | alt_inc_dec :
      forall (qm : queue Message) (s0 : Site),
      alternate qm ->
      alternate (input Message (inc_dec s0) (input Message dec qm)).Inductive alternate : queue Message -> Prop :=
  | alt_null : alternate (empty Message)
  | alt_single_inc :
      forall s0 : Site,
      alternate (input Message (inc_dec s0) (empty Message))
  | alt_any_alt :
      forall (qm : queue Message) (m : Message),
      (forall s : Site, m <> inc_dec s) ->
      alternate qm -> alternate (input Message m qm)
  | alt_inc_dec :
      forall (qm : queue Message) (s0 : Site),
      alternate qm ->
      alternate (input Message (inc_dec s0) (input Message dec qm)).Inductive alternate : queue Message -> Prop :=
  | alt_null : alternate (empty Message)
  | alt_single_inc :
      forall s0 : Site,
      alternate (input Message (inc_dec s0) (empty Message))
  | alt_any_alt :
      forall (qm : queue Message) (m : Message),
      (forall s : Site, m <> inc_dec s) ->
      alternate qm -> alternate (input Message m qm)
  | alt_inc_dec :
      forall (qm : queue Message) (s0 : Site),
      alternate qm ->
      alternate (input Message (inc_dec s0) (input Message dec qm)).Inductive alternate : queue Message -> Prop :=
  | alt_null : alternate (empty Message)
  | alt_single_inc :
      forall s0 : Site,
      alternate (input Message (inc_dec s0) (empty Message))
  | alt_any_alt :
      forall (qm : queue Message) (m : Message),
      (forall s : Site, m <> inc_dec s) ->
      alternate qm -> alternate (input Message m qm)
  | alt_inc_dec :
      forall (qm : queue Message) (s0 : Site),
      alternate qm ->
      alternate (input Message (inc_dec s0) (input Message dec qm)).

Lemma D_first_out : forall q0 : queue Message, D_queue q0 -> D_queue (first_out Message q0).