(map-entry? x)
Return true if x is a map entry
user=> (class {:a 1 :b 2})
clojure.lang.PersistentArrayMap
user=> (class (first {:a 1 :b 2}))
clojure.lang.MapEntry
;; A map entry is treated as an ordered collection of key and value.
;; https://github.com/clojure/clojure/blob/master/src/jvm/clojure/lang/MapEntry.java
user=> (map-entry? (first {:a 1 :b 2}))
true