  By using Newtonian mechanics, we construct a general model of Maxwell's
demon, a system in which the engine and the memory interact only through the
exchange of information. We show that the Jarzynski relation and the two
Sagawa-Ueda relations hold simultaneously, and argue that they are the unique
triplet which has a natural decomposition property. The uniqueness provides a
strong support to the assertion that the mutual information is the key
quantity.
