doc-src/Tutorial/Misc/Fib.thy

+Fib = WF_Rel +
+consts fib  :: nat => nat
+recdef fib "measure(%n. n)"
+    "fib 0 = 0"
+    "fib 1 = 1"
+    "fib (Suc(Suc x)) = fib x + fib (Suc x)"
+end