在Coq中定义Ackermann时出错

Error: Unable to satisfy the following constraints:
In environment:
  T: forall (A:Type) (b:A) (n:nat), AckermannType n A b = 1 / O
  P: forall (A:Type) (b:A) (n:nat), AckermannType n A b = 1 / O
  Q: forall (A:Type) (b:A) (n:nat), AckermannType n A b = 1 / O
  R: forall (A:Type) (b:A) (n:nat), AckermannType n A b = 1 / O
  ...
  where
    T, P, Q, R, ...: forall (A:Type) (b:A) (n:nat), AckermannType n A b = 1 / O

Definition Ackermann (A: Type) (n: nat) (b: A) : Prop :=
  match n with
  | 0 => True
  | S n' =>
      match b with
      | Const _ => Ackermann n' (b / a)
      | Var x => False
      end
  end.

Definition Ackermann (A: Type) (n: nat) (a: A) : Prop :=
  match n with
  | 0 => True
  | S n' =>
      match a with
      | Const _ => Ackermann n' (a / b)
      | Var x => False
      end
  end.