fix(library/logic/wf_k): missing file

library/logic/wf_k.lean

@@ -0,0 +1,17 @@
+-- Copyright (c) 2014 Microsoft Corporation. All rights reserved.
+-- Released under Apache 2.0 license as described in the file LICENSE.
+-- Author: Leonardo de Moura
+import logic.wf data.nat.basic
+
+namespace well_founded
+
+  -- This is an auxiliary definition that useful for generating a new "proof" for (well_founded R)
+  -- that allows us to use well_founded.fix and execute the definitions up to k nested recursive
+  -- calls without "computing" with the proofs in Hwf.
+  definition intro_k {A : Type} {R : A → A → Prop} (Hwf : well_founded R) (k : nat) : well_founded R :=
+  well_founded.intro
+  (nat.rec_on k
+     (λ n : A, well_founded.apply Hwf n)
+     (λ (k' : nat) (f : Πa, acc R a), (λ n : A, acc.intro n (λ y H, f y))))
+
+end well_founded