problem with vmcalculation plugins

[VampiRUS]

I have 2 different plugin with some mathop for discount.
problem situation:
i have 2 categories: cat 1 and cat 2
first plugin can to change price for products in both categories in some additional conditions, checked in plugin(must be selected necessary payment method).
second plugin can to change price only in cat 2, dependency on product count.
If i have in cart two products from different category, plugin 1 applied first, and returned new price. Any change from second plugin will be ignored.
Even if will be selected different payment method and i want in this case result from second plugin. I can return null from first plugin and in this case it will be good.
But when in cart only products from first category and some payment method selected(not from first plugin again) and plugin return null, i will recive warring "Unrecognised mathop some_math_op in calculation rule found, seems you created this rule with plugin not longer accesible (deactivated, uninstalled?)"


solved:
write another dummy plugin