(vector? x)
Return true if x implements IPersistentVector
;; this is the idiomatic vector
(vector? [1 2 3])
;;=> true
;; a list is not a vector
(vector? '(1 2 3))
;;=> false
;; a list may be converted into a vector
(vector? (vec '(1 2 3)))
;;=> true
;; a map is not a vector
(vector? {:a 1 :b 2 :c 3})
;;=> false
;; a set is not a vector
(vector? #{:a :b :c})
;;=> false
(first {:a 1 :b 2 :c 3})
;;=> [:c 3]
(vector? (first {:a 1 :b 2 :c 3}))
;;=> true