  If two control systems on manifolds of the same dimension are dynamic
equivalent, we prove that either they are static equivalent --i.e. equivalent
via a classical diffeomorphism-- or they are both ruled; for systems of
different dimensions, the one of higher dimension must ruled. A ruled system is
one whose equations define at each point in the state manifold, a ruled
submanifold of the tangent space. Dynamic equivalence is also known as
equivalence by endogenous dynamic feedback, or by a Lie-B\"acklund
transformation when control systems are viewed as underdetermined systems of
ordinary differential equations; it is very close to absolute equivalence for
Pfaffian systems. It was already known that a differentially flat system must
be ruled; this is a particular case of the present result, in which one of the
systems is assumed to be "trivial" (or linear controllable).
