(defun Y (le)
  ((lambda (f) (funcall f f))
   (lambda (f)
     (funcall le (lambda (x) (funcall (funcall f f) x))))))















#| Example call

(funcall (Y (lambda (fact) 
              (lambda (n) (if (< n 2) 1 (* n (funcall fact (1- n)))))))
         5)

|#