lean2/tests/lean/run/fib_wrec.lean

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import data.nat
open nat eq.ops
definition fib.F (n : nat) : (Π (m : nat), m < n → nat) → nat :=
nat.cases_on n
(λ (f : Π (m : nat), m < zero → nat), succ zero)
(λ (n₁ : nat), nat.cases_on n₁
(λ (f : Π (m : nat), m < (succ zero) → nat), succ zero)
(λ (n₂ : nat) (f : Π (m : nat), m < (succ (succ n₂)) → nat),
have l₁ : succ n₂ < succ (succ n₂), from lt.base (succ n₂),
have l₂ : n₂ < succ (succ n₂), from lt.trans (lt.base n₂) l₁,
f (succ n₂) l₁ + f n₂ l₂))
definition fib (n : nat) :=
well_founded.fix fib.F n
theorem fib.zero_eq : fib 0 = 1 :=
well_founded.fix_eq fib.F 0
theorem fib.one_eq : fib 1 = 1 :=
well_founded.fix_eq fib.F 1
theorem fib.succ_succ_eq (n : nat) : fib (succ (succ n)) = fib (succ n) + fib n :=
well_founded.fix_eq fib.F (succ (succ n))
example : fib 5 = 8 :=
rfl
example : fib 6 = 13 :=
rfl