title: How to define xvalue mixed category?
body: <p>Are all <code>xvalues</code> <code>glvalues</code> and <code>rvalues</code> at the same time? Or a <code>xvalue</code> may be either <code>glvalue</code> or a <code>rvalue</code>?</p>
  
  <p>If it's a <code>glvalue</code> or/xor <code>rvalue</code>, can you give a example for each case?</p>