def FunType := Nat → Nat
def Fun2Type := Nat → Nat → Nat

mutual
def foo : FunType
  | .zero => 0
  | .succ n => bar n
termination_by n => n
def bar : Nat → Nat
  | .zero => 0
  | .succ n => baz n 0
termination_by n => n
def baz : Fun2Type
  | .zero, m => 0
  | .succ n, m => foo n
termination_by n _ => n
end