=item my $prefix = shortest_prefix @strings;

Return the shortest prefix common to all @strings.  Returns empty
string if there is no prefix; returns C<undef> if there are no strings
or if any of the values are themselves C<undef>

=cut

# algorithm by Ilya Z
# implementation by Ken Fox
# both ganked from c.l.p.misc:
# http://groups.google.fm/groups?selm=199908100340.UAA13944%40long-lake.nihongo.org

sub common_prefix ( @ )
{
    defined( my $prefix = shift )   or return undef;

    my $len = length $prefix        or return '';

    foreach my $w ( @_ )
    {
        defined $w                  or return undef;

        while ( substr( $w, 0, $len ) ne $prefix )
        {
            --$len;
            chop $prefix;
        }

        $len                        or return '';
    }

    return $prefix;
}