CenterStageObject Class:  CurveFromSurface
Subclass of:  Curve
Requires reference to a Surface object

The CurveFromSurface lets you create a Curve object that relates to
a Surface object.  You specify a parametric curve in the domain of the
surface, and that curve is computed in space.  Then the second part of
the curve function is computed, and its result is added to the value
of the point on the surface, either as as absolute coordinates, or in
relation to a frame on the surface.  The frame is determined by the
normal vector to the surface and the partial derivatives of the
surface with respect to its parameters (note that this need not be an
orthonormal basis).

In addition to the standard Curve directives, CurveFromSurface supports
the following:

   Function param curve func [-frame | -absolute]

	Here the "param" and "func" arguments are the same as for the
	standard Curve.  The -frame and -absolute options determine
	whether the result of the function is in the coordinate system
	of the surface frame or in absolute coordinates (the default).
	If -frame is provided, then the surface and curve must be in
	3-space, and the frame is computed numerically.  The
	x-coordinate of the result corresponds to the normal
	direction for the surface, the y-coordinate to the direction
	of the first partial derivative of the surface, and the
	z-coordinate to the direction of the second partial
	derivative.

	The "curve" argument is a script that gives a curve within the
	surface's domain parametricly.

	For example:

		Function {t} {
		  let (x,y) = (t,t^2)
                } {
		  let (x,y,z) = (.1,0,0)
		} -frame

	produces a curve that is offset from the surface by .1 in the
	normal direction along the curve that is the image of a
	parabola in the domain of the surface.