even?  is a predicate that test if the parameter is an even number, as the name suggests. ? is part of the function name, not a syntax symbol (there&#x27;s not much syntax in a Lisp, after all). By convention, function name ends with ? are predicates (or Boolean functions) in Clojure.<p>Any competent Clojure programmer would likely have known most of the Clojure core functions by heart: map, reduce, filter, etc. There are maybe about a hundred of them?<p>Unlike imperative programming languages, where you use three primitives (assignment, condition and loop) to construct everything, in Clojure, you use these core functions to construct your program.  In a sense, the process of learning Clojure is the process of learning the use of core functions.  These functions are simply at a higher level of abstraction than the three primitives, hence some people say that Clojure is more expressive, because it&#x27;s closer to human thoughts than Von Neumann machine specificity.