open nat prod sigma

-- We will define the following example by well-foudned recursion
--   g 0        := 0
--   g (succ x) := g (g x)
definition g.F (x : nat) : (Π y, y < x → Σ r : nat, r ≤ y) → Σ r : nat, r ≤ x :=
nat.cases_on x
  (λ f, ⟨zero, le.refl zero⟩)
  (λ x₁ (f : Π y, y < succ x₁ → Σ r : nat, r ≤ y),
     let p₁    := f x₁ (lt.base x₁) in
     let gx₁   := pr₁ p₁ in
     let p₂    := f gx₁ (lt_of_le_of_lt (pr₂ p₁) (lt.base x₁)) in
     let ggx₁  := pr₁ p₂ in
     ⟨ggx₁, le_succ_of_le (le.trans (pr₂ p₂) (pr₂ p₁))⟩)

definition g (x : nat) : nat :=
pr₁ (well_founded.fix g.F x)

example : g 3 = 0 :=
rfl

example : g 6 = 0 :=
rfl

theorem g_zero : g 0 = 0 :=
rfl

theorem g_succ (a : nat) : g (succ a) = g (g a) :=
have aux : well_founded.fix g.F (succ a) = sigma.mk (g (g a)) _, from
  well_founded.fix_eq g.F (succ a),
calc g (succ a) = pr₁ (well_founded.fix g.F (succ a)) : rfl
          ...   = pr₁ (sigma.mk (g (g a)) _)          : aux
          ...   = g (g a)                              : rfl

theorem g_all_zero (a : nat) : g a = zero :=
nat.induction_on a
  g_zero
  (λ a₁ (ih : g a₁ = zero), calc
     g (succ a₁) = g (g a₁) : g_succ
          ...    = g 0      : ih
          ...    = 0        : g_zero)