title: Parameterized Array#uniq (i.e., uniq_by)
body: <p>What if instead of removing duplicate elements from an array, I want to remove elements that have a specific property in common?</p>
  
  <p>Specifically, I want to remove all strings from an array with duplicate "essences", where essence is defined like this:</p>
  
  <pre><code>class String
    def essence
      downcase.gsub('&amp;', 'and').gsub(/[^a-z0-9]/, '')
    end
  end
  </code></pre>
  
  <p>I want something like this:</p>
  
  <pre><code>['a', 'A', 'b'].uniq_by(&amp;:essence)
  # =&gt; ['a', 'b'] (or ['A', 'b']; I don't really care)
  </code></pre>
  
  <p>What's the best way to accomplish this?</p>