sub FindCommonPrefix {
    my $model= pop @_;
    my $len= length($model);
    for my $item (  @_  ) {
        my $dif= $model ^ substr($item,0,$len);
        $len= length( ( $dif =~ /^(\0+)/ )[0] );
        substr( $model, $len )= "";
    }
    return $model;
}