/--
error: tactic 'fail' failed
z : Nat
⊢ z - 1 < z
-/
#guard_msgs in
def g (z : Nat) (y : Nat) : Nat := g (z - 1) y
termination_by z
decreasing_by fail

/--
error: tactic 'fail' failed
x : List Nat
⊢ sizeOf x.tail < sizeOf x
-/
#guard_msgs in
def h (x : List Nat) (y : Nat) : Nat := h x.tail y
termination_by x
decreasing_by fail

/--
error: tactic 'fail' failed
x : List Nat
⊢ x.tail.length < x.length
-/
#guard_msgs in
def f (x : List Nat) (y : Nat) : Nat := f x.tail y
termination_by x.length
decreasing_by fail

/--
error: tactic 'fail' failed
x : List Nat
⊢ x.tail.length < x.length
-/
#guard_msgs in
mutual
def f1 (x : List Nat) (y : Nat) : Nat := f2 x.tail y
termination_by x.length
decreasing_by fail
def f2 (x : List Nat) (y : Nat) : Nat := f1 x.tail y
termination_by x.length
decreasing_by fail
end

/--
error: tactic 'fail' failed
x : List Nat
⊢ (invImage (fun x => PSum.casesOn x (fun x => x.length) fun x => x.length) instWellFoundedRelationOfSizeOf).1
    (PSum.inr x.tail) (PSum.inl x)
-/
#guard_msgs in
set_option debug.rawDecreasingByGoal true in
mutual
def g1 (x : List Nat) (y : Nat) : Nat := g2 x.tail y
termination_by x.length
decreasing_by fail
def g2 (x : List Nat) (y : Nat) : Nat := g1 x.tail y
termination_by x.length
decreasing_by fail
end