title: How to substract 1 from bidirectional iterator
body: <p>I have iterator I dont want to change, but I would like to assign another iterator to the value of that iterator -1, so<br>
  <code>it2 = --it1;</code>
  is out of the question.<br>
  Problem:<br>
  <code>it2 = it1-1;</code>
  doesnt work.</p>
  
  <blockquote>
    <p>error: no match for 'operator-' in 'bigger - 1'
    c++/4.1.1/bits/stl_bvector.h:182: note: candidates are: ptrdiff_t
    std::operator-(const std::_Bit_iterator_base&amp;, const
    std::_Bit_iterator_base&amp;)</p>
  </blockquote>
  
  <p>ofc I could do </p>
  
  <pre><code>it2 = --it1;
  it1++;
  </code></pre>
  
  <p>but that is horrible. </p>