title: signed NaN values
body: <p>Based on "IEEE" spec :
  "When either an input or result is NaN, this standard does not interpret the sign of a NaN."
  However the <code>printf</code> prints <code>NaN</code> values as signed:<code>nan</code> or <code>-nan</code>
  Can someone point me the set of rules(from spec?) when <code>nan</code> and when <code>-nan</code> is printed
  For example , I checked that <code>printf(-1.0f)</code> prints <code>-nan</code>
  Thank you</p>