title: List comprehension of 2+ variables in R
body: <p>What's the best R equivalent of the Python 2-variable list comprehension</p>
  
  <pre><code>[datetime(y,m,15) for y in xrange(2000,2020) for m in [3,6,9,12]]
  </code></pre>
  
  <p>The result</p>
  
  <pre><code>[datetime.datetime(2000, 3, 15, 0, 0), 
  datetime.datetime(2000, 6, 15, 0, 0), 
  datetime.datetime(2000, 9, 15, 0, 0),
  datetime.datetime(2000, 12, 15, 0, 0), 
  datetime.datetime(2001, 3, 15, 0, 0) ... ]
  </code></pre>