  It is ``folklore'' that the solution to a set reachability problem for a
dynamical system is only noncomputable because of non-robustness reasons. A
robustness condition that can be imposed on a dynamical system is the
requirement of the chain reachable set to equal the closure of the reachable
set. We claim that this condition necessarily imposes strong conditions on the
dynamical system. For instance, if the space is connected and compact and we
are computing a chain reachable robust single valued function $f$ then $f$
cannot have an unstable fixed point or unstable periodic cycle.
