title: boost::icl::interval_map union on overlap
body: <p>Is it possible to make <a href="http://www.boost.org/doc/libs/1_61_0/libs/icl/doc/html/boost/icl/interval_map.html" rel="nofollow noreferrer">interval_map</a> with following accumulative behavior:</p>
  
  <pre><code>[1..3]-&gt;1 +
  [2..4]-&gt;2 +
  [5..7]-&gt;3 +
  [6..8]-&gt;4
  =
  [1..4]-&gt;{1,2}, [5..8]-&gt;{3,4}
  +
  [3..6]-&gt;42
  =
  [1..8]-&gt;{1,2,3,4,42}
  </code></pre>