  This paper generalises results of Willems-Trentelman, and van der Schaft, on
achievable behaviours, to the case of linear distributed systems defined by
partial differential or difference equations. It shows that the `minimal'
controller which achieves a particular subsystem is the canonical controller of
van der Schaft, thereby answering the `open problem' of \cite{sc} in the
setting of infinite dimensional and $n-D$ systems. This result is used to
describe the collection of all linear subsystems of the electro-magnetic field,
containing the vacuum solutions, that can be achieved by suitable choices of
electric charge and current density.
