Fixpoint eq_nat (m n: nat) :=
  match m, n with (S m), (S n) => eq_nat m n | 0, 0 => true | _, _ => false end.Notation "m ?= n" := (eq_nat m n) (at level 70): nat_scope.Inductive eq_nat_spec: nat -> nat -> bool -> Type := 
  eq_nat_spect: forall m, eq_nat_spec m m true
| eq_nat_specb: forall m n, m <> n -> eq_nat_spec m n false.Inductive eq_nat_spec: nat -> nat -> bool -> Type := 
  eq_nat_spect: forall m, eq_nat_spec m m true
| eq_nat_specb: forall m n, m <> n -> eq_nat_spec m n false.Inductive eq_nat_spec: nat -> nat -> bool -> Type := 
  eq_nat_spect: forall m, eq_nat_spec m m true
| eq_nat_specb: forall m n, m <> n -> eq_nat_spec m n false.

Lemma eq_natP m n: eq_nat_spec m n (m ?= n).