I have a simple task:
Given a class, I need a function that can add elements to one of the
fields of the class.
It's easy to do if the function knows the name of the field, but what
if the function doesn't know it and we need to pass it as an input
parameter to the function.
Let's say
(defclass test1 ()
  ((fld1 :accessor test1-fld1 :initarg :fld1 :type list)))

(setq o (make-instance 'test1 :fld1 nil))
(defun append-element (o el)
  (with-slots (fld1) o
    (setf fld1 (append fld1 (list el)))))