Activation Functions in Lisp

Every so often I get an urge to write some Lisp code. I do not have much experience with Lisp. I need to think of a project I can take on that would excite me! Here are some activation functions I wrote in Lisp earlier:

(defun sigmoid (a) (/ 1 (+ 1 (exp (- (abs a))))))

(defun baserelu (a retr) (if (> a 0) a retr))

(defun relu (a) (baserelu a 0))

(defun leakyrelu (a) (baserelu a 0.01))

To write these functions, I used the CommonLisp programming environment in Repl.it. ^1 This was much easier than researching how to run a Lisp program on my local machine again.