Entries in the panel window can be classified into two categories:
object creation/modification/removal commands (only one of which may be active
at any one time), and drawing aids (which act as toggle switches).
There are two ways for drawing circles, two for ellipses, two for splines
and two for closed splines.
There are two basic splines. One is the interpolated spline
in which the spline pass thorough the entered points (knots).
The other is the normal spline in which on control points are
passed by the spline (except for the two end points in the open spline).