s <tt>b.clear()</tt>
then attempts to extract characters from <tt>is</tt>. For each character c
that is a <i>bitset digit</i> the <i>corresponding bit value</i> is
appended to the less significant end of <tt>b</tt> (appending may throw).
If <tt>is.width()</tt> is greater than zero and smaller than <tt>b.max_size()</tt>
then the maximum number <tt>n</tt> of bits appended is <tt>is.width()</tt>;
otherwise <tt>n</tt> = <tt>b.max_size()</tt>.

Unless the extractor is exited via an exception, characters are extr