feat: 9.5.9

examples/fatfib.ss

@@ -0,0 +1,19 @@
+;;; fat fibonacci function
+
+;;; this is "fat" because it uses only increments and decrements
+;;; for addition and subtraction (i.e., peano arithmetic).
+
+;;; note that fat+ is tail-recursive; this is how all looping is
+;;; performed in Scheme.
+
+(define fat+
+   (lambda (x y)
+      (if (zero? y)
+          x
+          (fat+ (1+ x) (1- y)))))
+
+(define fatfib
+   (lambda (x)
+      (if (< x 2)
+          1
+          (fat+ (fatfib (1- x)) (fatfib (1- (1- x)))))))