Thank you for brilliant script - it's perfect. Really.

Dear Tom, please, can you recommend me how i must to do to plusing some field. For example:

<?php echo $hm->Zb('rs:def:some_digits'); ?>
+
<?php echo $hm->Zb('rs:def:some_digits2'); ?>
=
<?php echo $hm->Zb('rs:def:some_digits3'); ?>

Thank you for fast reply.

No, you can't combine Zb(...) like that. you have to create a new field class to combine multiple input tags.