mutually recursive

A set of program units which call each other. For example, function A calls function B, which then recursively calls A again, and so on (until, hopefully, a base case is reached). This technique is called mutual recursion.

(define even?
  (lambda (n)
    (if (zero? n)
        #t
        (odd? (- n 1)))))

(define odd?
  (lambda (n)
    (if (zero? n)
        #f
        (even? (- n 1)))))