anchor.diff:  [source,clojure-repl]
   ----
   $ clj
  -Clojure 1.9.0
  +Clojure 1.10.0
   user=> (ns myproject.person-names)
   nil
   myproject.person-names=> nicknames ;; #'nicknames won't be defined, because the lib has not been loaded.
  -CompilerException java.lang.RuntimeException: Unable to resolve symbol: nicknames in this context, compiling:(NO_SOURCE_PATH:0:0)
  +Syntax error compiling at (REPL:0:0).
  +Unable to resolve symbol: nicknames in this context
   myproject.person-names=> (require '[myproject.person-names]) ;; won't fix the situation, because the namespaces has already been created
   nil
   myproject.person-names=> nicknames
  -CompilerException java.lang.RuntimeException: Unable to resolve symbol: nicknames in this context, compiling:(NO_SOURCE_PATH:0:0)
  -----
  +Syntax error compiling at (REPL:0:0).
  +Unable to resolve symbol: nicknames in this context
  +----
  
positive.code: [source,clojure-repl]
  ----
  $ clj
  Clojure 1.9.0
  user=> (in-ns 'myapp.never-created)
  #object[clojure.lang.Namespace 0x22356acd "myapp.never-created"]
  myapp.never-created=> (defn say-hello [x] (println "Hello, " x "!"))
  CompilerException java.lang.RuntimeException: Unable to resolve symbol: defn in this context, compiling:(NO_SOURCE_PATH:2:1)
  ----
  
  **Explanation:** in this situation, https://clojure.github.io/clojure/clojure.core-api.html#clojure.core/in-ns[in-ns]
  
negative.code: [source,clojure-repl]
  ----
  $ clj
  Clojure 1.10.0
  user=> (ns myapp.foo.bar) ;; creating and switching to the `myapp.foo.bar` namespace - NOTE `ns` will be explained later in this guide
  nil
  myapp.foo.bar=> (def my-favorite-number 42) ;; defining a Var named `my-favorite-number`
  