  We consider the category of partially observable dynamical systems, to which
the entropy theory of dynamical systems extends functorially. This leads us to
introduce quotient-topological entropy. We discuss the structure that emerges.
We show how quotient entropy can be explicitly computed by symbolic coding. To
do so, we make use of the relationship between the category of dynamical
systems and the category of graphs, a connection mediated by Markov partitions
and topological Markov chains.
