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transitions. The extent to which task-based approaches have been used before in security sys-
tems has also been discussed.
The two protocols and the relationship between them are defined in Petri-Nets. The overall
model is formally defined using a categorical pullback construction. Each of the protocols,
represented as Petri-Nets for state-transition purposes, is a category-valued functor in the pull-
back. The use of Petri-Nets within a categorical framework looks to be a promising way for-
ward for security problems.
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