title: Double Iteration in List Comprehension
body: <p>In Python you can have multiple iterators in a list comprehension, like</p>
  
  <pre><code>[(x,y) for x in a for y in b]
  </code></pre>
  
  <p>for some suitable sequences a and b. I'm aware of the nested loop semantics of Python's list comprehensions.</p>
  
  <p>My question is: Can one iterator in the comprehension refer to the other? In other words: Could I have something like this:</p>
  
  <pre><code>[x for x in a for a in b]
  </code></pre>
  
  <p>where the current value of the outer loop is the iterator of the inner?</p>
  
  <p>As an example, if I have a nested list:</p>
  
  <pre><code>a=[[1,2],[3,4]]
  </code></pre>
  
  <p>what would the list comprehension expression be to achieve this result:</p>
  
  <pre><code>[1,2,3,4]
  </code></pre>
  
  <p>?? (Please only list comprehension answers, since this is what I want to find out).</p>