This is handy to add some elements to configuration variables,
but please do not abuse it in Elisp code, where you are usually
better off using `push' or `cl-pushnew'.

(setq testasdf nil)

(push 'a testasdf)

testasdf
(a)

(add-to-list 'testasdf 'b)

testasdf
(b a)

;; add element to the end
(add-to-list 'testasdf "hello" t)

testasdf
(b a "hello")