If o is a vertex, set its :z value to zero; else throw an exception.

Given a vertex o, if o has a :z value, just return o; otherwise return a vertex like o but having thie dflt value as the value of its :z key, or zero as the value of its :z key if dflt is not specified.

If o is not a vertex, throws an exception.

True if o satisfies the conditions for a vertex. That is, essentially, that it must rerpresent a two- or three- dimensional vector. A vertex is shall be a map having at least the keys :x and :y, where the value of those keys is a number. If the key :z is also present, its value must also be a number.

The name vector? was not used as that would clash with a function of that name in clojure.core whose semantics are entirely different.