1. A device for calculating a post-image in a system, the device comprising: a first store arranged to store bits representative of transition functions of the system; a second store arranged to store bits representative of an estimate of transition functions of a reverse model; a processing system constructed and arranged to form transition functions of said reverse model without knowing input states and corresponding output states of the system, said processing system comprising a logical device constructed and arranged to transform said transition functions of said system into constraints on said reverse model; and a parameterization processor constructed and arranged to apply a parameterization of said constraints to said estimate of transition functions of said reverse model to form said transition functions of said reverse model; a third store arranged to store bits representative of the transition functions of said reverse model of said system; a fourth store arranged to store bits representative of a set of states of said system; and a forming device constructed and arranged to substitute the state variables of the reverse model by the transition functions of said reverse model to provide a new set of states representing a pre-image of said reverse model, and thus the post-image in said system.