title: log2 in python math module
body: <p>why doesn't it exist?</p>
  
  <pre><code>import math
  [x for x in dir(math) if 'log' in x]
  &gt;&gt;&gt; ['log', 'log10', 'log1p']
  </code></pre>
  
  <p>I know I can do log(x,2), but log2 is really common, so I'm kind of baffled.</p>
  
  <p>Oh, it looks like it's only defined in C99, not C90, I guess that answers my question. Still seems kind of silly.</p>