title: Elegant summation of list comprehension
body: <p>List comprehensions are great. Is there an elegant, slick way to do a list comprehension that <em>appends</em> items to each other as they go, rather than putting them all in a list?</p>
  <p>IE, I have some comprehension <code>[func(x) for x in y if z]</code> that spits out <code>[['a','b'],['c','d'],['e','f']]</code>, what can i write to instead spit out <code>['a','b','c','d','e','f']</code>?</p>
  <p>Obviously I could do</p>
  <pre><code>alist = []
  
  for x in y:
      if z:
          alist += func(x)
  </code></pre>
  <p>but that's four or five lines of code! it feels like there must be a way to do it that's as simple and easy as the list comprehension is.</p>