title: is it safe to add a value to a std::deque iterator
body: <p>std::deque unlike std::vector is not always contiguous in memory.</p>
  
  <p>is it safe, for example to do this :</p>
  
  <pre><code>std::size_t pos;
  //... compute pos ...
  mydeque.erase(mydeque.begin(), mydeque.begin() + pos);
  </code></pre>
  
  <p>knowing that "mydeque.begin() + pos", is in the deque range ?</p>
  
  <p>I read that the iterator is "RandomAccessIterator" so normally it's safe, am I right ?</p>