  Starting from a simple marginally stable model considered for Lyapunov based
boundary control of flexible mechanical systems, we add a term driving an
instability and prove that for an appropriate control condition the system can
become Lyapunov stable. A similar approximate extension is found for the
general energy principle of linearized magnetohydrodynamics. The implementation
of such external instantaneous actions may, however, impose challenging
constraints for fusion plasmas.
