anchor.diff:  There are three ways to build up custom generators - in decreasing order of preference: 
   
   . Let spec create a generator based on a predicate/spec
  -. Create your own generator from the tools in clojure.spec.gen
  +. Create your own generator from the tools in clojure.spec.gen.alpha
   . Use test.check or other test.check compatible libraries (like https://github.com/gfredericks/test.chuck[test.chuck])
   
   [WARNING]
  
positive.code: 
  == Getting started
  
  The <<xref/../../../about/spec#,spec>> library specifies the structure of data, validates or destructures it, and can generate data based on the spec. The https://clojure.github.io/clojure/branch-master/clojure.spec-api.html[clojure.spec] namespace is included in the Clojure core distribution, so no extra library is required to use it. You will need to declare a dependency on the latest alpha version of Clojure (or higher) however:
  
  [source, clojure]
  ----
  [org.clojure/clojure "1.9.0-alpha15"]
  ----
  
  To start working with spec, require the `clojure.spec` namespace at the REPL:
  
  [source,clojure]
  ----
  (require '[clojure.spec :as s])
  ----
  
  Or include spec in your namespace:
  
negative.code: [source,clojure]
  ----
  (ns my.ns
    (:require [clojure.spec.alpha :as s]))
  ----
  
  == Predicates
  