This demo displays the circle given by rational parametrization X(t) =
((1 - t^{2})/(1 + t^{2}), 2t/(1 + t^{2})). To move the
point X(t_{0}) along the line, use the following keys on the
appropriate line in the control window:

Change the value of t_{0} as in earlier demos.

Check the box "Angle paramter" to see some geometric properties of this
parametrization.

As in earlier demos, there are "Show entire curve", "Show position
vector X(t0)" and "Show Color Spectrum" options.