  We use categorical methods to define a new flavor of Petri nets where
transitions can only fire a limited number of times, specified by a quantity
that we call mana. We do so with chemistry in mind, looking at ways of
modelling the behavior of chemical reactions that depend on enzymes to work. We
prove that such nets can be either obtained as a result of a comonadic
construction, or by enriching them with extra information encoded into a
functor. We then use a well-established categorical result to prove that the
two constructions are equivalent, and generalize them to the case where the
firing of some transitions can "regenerate" the mana of others. This allows us
to represent the action of catalysts and also of biochemical processes where
the byproducts of some chemical reaction are exactly the enzymes that another
reaction needs to work.
