(contains? {a null} a)
(contains? <!a> (jsonptr "/a"))
(contains? {a {b 1}} [a b])
(contains? [a b] 1)
(contains? {a 1} missing)
(contains? {a {}} [a b])
(contains? [a b] 2)
(contains? {a/b null} a/b)
(contains? <!/a> ["/a"])
(contains? <!/a> (jsonptr "/~1a"))
(contains? {users [{name null}]} [users 0 name])
(contains? {users [{name null}]} (jsonptr "/users/0/name"))
(contains? {"" null} "")
(contains? {} [])
(contains? [] [])
(contains? null a)