anchor.diff:  optimizations. While advanced optimizations does apply performance
   related optimizations, these are now largely obviated by optimizations
   present in modern JavaScript virtual machines like V8, SpiderMonkey, and
  -JavaScriptCore. For Node.js, `:simple` or `:none` optimizations suffice
  -and using them removes the need for extra steps like supplying an
  -externs file.
  +JavaScriptCore. For Node.js, `simple` or `none` optimizations suffice.
   ====
   
   [[node.js-repl]]
  
positive.code: `-m`, which is short for `--main` invokes a Clojure function, in this case
  `cljs.main`. The `cljs.main` function supports a variety of command line
  arguments to specify common tasks. We're using `-c` for `--compile` to specify
  that we want to compile the `hello-world.core` name space. This is followed by
  `-r` for `--repl` to say that want a REPL to start immediately when compilation
  completes.
  
  Your default web browser will open to a page that looks like the following:
  
negative.code: == Production Builds
  
  You may have noticed the `out` directory which contains all of the compiled
  JavaScript. There's about 6 1/2 megabytes worth of JavaScript in there. This may
  seem unweildy but fortunately the ClojureScript compiler generates output
  optimized for the Google Closure Compiler. The Google Closure Compiler performs
  many optimizations, but the most significant for browser-based clients are
  minification and dead code elimination.
  
  Let's remove the dev time REPL bits from `src/hello_world/core.cljs`:
  
  