So far, every nonholonomic constraint I have seen can be expressed as a collection of inequalities involving the coordinates of the system. For example, a small ball rolling down a sphere with radius a has the constraint [itex]r^2-a^2\geq 0[/itex], where r is the radial coordinate of the ball. Can every nonholonomic constraint be written in this form? If not, I would appreciate a counterexample. Thanks in advance.