title: PyYAML customized yaml processing
body: <p>I would like to extend YAML with some custom macros so that I can "reuse" parts of definitions within the same file. Sample:</p>
  
  <pre><code>DEFAULTS:
  - a
  - b
  - c
  CUSTOM1:
  - %DEFAULTS
  - d
  CUSTOM2:
  - %DEFAULTS
  - e
  </code></pre>
  
  <p>resulting in </p>
  
  <pre><code>CUSTOM1==['a','b','c','d']
  CUSTOM2==['a','b','c','e']
  </code></pre>
  
  <p>Doesn't have to be exact same syntax, as long as I can get same functionality out of it. what are my options?</p>
  
  <p>P.S. 
  I do realize that it is possible to just walk the dictionary after parsing and re-adjust the values, however I'd like to do it while loading.</p>