title: Concatenate list elements that fall between list elements of certain value
body: <p>Imagine I have the following list:</p>
  
  <pre><code>&gt;&gt;&gt; mylist
  
  [('a', u'DT'),
   ('Satisfactory', u'JJ'),
   ('tracing', u'VBG'),
   ('with', u'IN'),
   ('a', u'DT'),
   ('fairly', u'RB'),
   ('persistent', u'JJ'),
   ('with', u'IN')]
  </code></pre>
  
  <p>How do I concatenate list items that fall between elements that contain <code>u'IN'</code> or <code>u'DT'</code> and return only the concatenated elements i.e:</p>
  
  <pre><code>[('Satisfactory tracing'),
   ('fairly persistent')]
  </code></pre>