*

test.grr

@@ -1,15 +1,13 @@
-def over { swap dup >r swap r> }
 def dip  { swap >r call r> }
 def keep { over >r call r> }
 def if   { ? call }
 
-def fac {
-  dup if: 1 <= [drop 1] [dup 1 - fac *];
+def fib/aux {
+  if: dig dup 0 =
+    [drop drop]
+    [bury [swap 1 - swap] dip dup [+] dip swap fib/aux]
+  ;
 }
+def fib { 0 1 fib/aux }
 
-def fib {
-  dup if: 1 <= [] [dup 1 - fib swap 2 - fib +];
-}
-
-[ 20 fib ] call \=> 6765
-[ 10 fac ] call \=> 3628800
+[ 50 fib ] call \=> 12586269025