title: Why we don't have 'const iterator' in the C++ standard, but const_iterator?
body: <p>Why was a special type <code>const_iterator</code> introduced?</p>
  
  <p>Why not make some of the iterator internals mutable, and go with standard C++ <code>const iterator</code>?</p>
  
  <p>Was <code>const iterator</code> ever discussed?</p>