  Choreography-based approaches to service composition typically assume that,
after a set of services has been found which correctly play the roles
prescribed by the choreography, each service respects his role. Honest services
are not protected against adversaries. We propose a model for contracts based
on a extension of Petri nets, which allows services to protect themselves while
still realizing the choreography. We relate this model with Propositional
Contract Logic, by showing a translation of formulae into our Petri nets which
preserves the logical notion of agreement, and allows for compositional
verification.
