2D CLSD allows you to create a 2-D cubic parametric spline
(which lies within a plane) that starts and ends at the
first point selected, appearing as an unbroken loop. The
slope and curvature are continuous at all points on the
spline.

Indicate a series of node points for the new spline, using
Position Menu options.

In 2-D construction mode, the nodes are projected to the
current construction plane, and the spline is created in 
the current construction plane.

In 3-D construction mode, the system tries to define a 
plane with the node points. If a plane can be defined, the
spline is created in this plane. If a plane cannot be
defined with the indicated node points, the points are
projected to the current construction plane, and the 
spline is created in the current construction plane.
