Can you please debug this lisp recursive function for me LINE BY LINE?

(defun SAFE-SUM L) (if (endp L) 0 (let ((X (SAFE-SUM (CDR L)))) (if (and (numbers (car L)) (numberp x)) (+ (car L) X) ERROR)))))

L = (4 1 6 3) X = 10

I already know the answer. Just when I debug it on paper, I'm not following how the recursive function is being called. Is ((X (SAFE-SUM (cdr L)))) executed repeatedly first or after the first call, it executes the next lines and then call itself again? I'm kind of confused about that. Thank in advance.