CenterStage Object Class:  RuledSurface
Subclass of:  SurfaceFromCurve
Requires reference to a Curve object

The RuledSurface class builds a ruled surface from a given Curve
object using a function that determines the ruling direction at each
point along the curve.  This direction can be given in terms of
aboslute coordinates, or in terms of the Frenet frame for the curve.

The first parameter in the domain for the ruled surface represents the
parameter along the rulings, while the second represents the position
along the curve.

The RuledSurface class supports all the directives of the
SurfaceFromCurve class (other than the Function directive), plus the
following:

    Vector param func [-frame | -absolute]

	Here the "param" is the same as in the Curve class's Function
	directive, and the "func" is a TCL script whose result is
	interpreted as a vector that determines the direction of the
	ruling for the surface at this parameter value.  If -frame is
	specified, the vector is in terms of the Frenet frame (i.e.,
	the x, y and z coordinates give distances along the tangent,
	normal and binormal directions), while if -absolute is
	specified (the default), then the vector's coordinates are in
	the coordinates of the original curve.

	The default Vector function yields the tangent developable
	surface for the given curve.  The normal and binormal
	developable surfaces are also easily produced.

	For a plane curve, the normal developable can be used to view
	normal lines to the original curve (using the DOMAIN/LINES
	menu and selecting the first parameter) or a series of offset
	curves to the original curve (by selecting the second
	parameter in the DOMAIN/LINES menu).