title: Is there any multiple-inheritance code in the C++ standard library or in Boost?
body: <p>Is there any code in the:</p>
  
  <ol>
  <li>C++ standard library (as of C++14, or in the final draft of C++17) </li>
  <li>Boost libraries </li>
  </ol>
  
  <p>that uses multiple inheritance? </p>
  
  <p>(I'm interested in this fact in the context of forming a mental picture of where multiple inheritance "stands" in the C++ world these days.)</p>