title: std::vector resize downward
body: <p>The C++ standard seems to make no statement regarding side-effects on capacity by either 
  <code>resize(n)</code>, with <code>n &lt; size()</code>, or <code>clear()</code>.</p>
  
  <p>It does make a statement about amortized cost of <code>push_back</code> and <code>pop_back</code> - O(1)</p>
  
  <p>I can envision an implementation that does the usual sort of capacity changes 
  ala CLRS Algorithms (e.g. double when enlarging, halve when decreasing <code>size to &lt; capacity()/4</code>).
  (Cormen Lieserson Rivest Stein)</p>
  
  <p>Does anyone have a reference for any implementation restrictions?</p>