sub compute {
    my( $opsub, $x, $y, $rev )= @_;
    $y= $x->new( $y )
        if  ! ref $y;
    ( $x, $y )= ( $y, $x )
        if  $rev;
    return $opsub->($x,$y);
}

##</code><code>##

sub mul {
    my $mulsub = sub { bless \( $$_[0] + $$_[1] ) };
    return &compute($mulsub,@_);
}