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

The SurfaceFromCurve lets you create a Surface object that relates to
an existing curve object.  The second parameter for the surface is used as 
the parameter for the curve, and at each value of the parameter, the curve 
function is evaluated.  Then for each value of the other parameter, the 
surface function is computed, and the two are added, either in absolute 
coordinates, or in terms of the Frenet frame of the curve.

In addition to the standard Surface directives, SurfaceFromCurve supports
the following:

   Function params func [vfunc] [-frame | -sframe | -absolute]

	Here the "params", "func" and "vfunc" arguments are the same as for
	the standard Surface.  The -frame, -sframe and -absolute options
	determine whether the result of the function is in the coordinate
	system of the Frenet frame for the reference curve, the surface
	frame of the linked curve (for CurveOnSurface objects) or in
	absolute coordinates (the default).  If -frame or -sframe is
	specified, then both the surface and the curve must be in 3-space,
	and the frame is computed numerically.  For -frame, the x, y and z
	coordinates correspond to the tangent, normal and binormal
	directions for the linked curve; for -sframe, they correspond to
	the surface normal and the parametric directions with respect to
	the first and second variables for the curve's linked surface.

	If only one of the parameters is specified for the function, the
	second is taken from the reference curve.

        For example:

                Function {s} {
                  let (T,N,B) = r (0,cos s,sin s)
                } -frame

                Axes {T N B}

                Slider r 0 2 .1

        produces a circular tube centered around the reference curve.  The 
        radius is controlled by a slider, so it is easy to investigate the 
        family of tubes around the curve.


    Domain {{umin umax udivs} {Inherit}}
        
	This special form of the domain command can be used to inherit the 
	domain from the original curve.  Standard Domain specifications can 
	also be used to provide a domain other than the one of the original 
	curve.