title: Homebrew: List only installed top level formulas
body: <p>I'm looking for a way to show only the formulas I installed without the installed dependencies.
  I want to have a list of all the programs I actually installed, without all noise of the dependencies.</p>
  
  <p>I do know about <code>brew list</code> which lists all installed formulas.
  I also know that <code>brew graph</code> gives me a dependency graph in the <code>graphviz</code></p>
  
  <p>Or in other words: I want to have the minimal set of formulas to reinstall my system.</p>