gluBeginTrim,	gluEndTrim - delimit a NURBS trimming loop

	  void gluBeginTrim( GLUnurbs* nurb )

	  void gluEndTrim( GLUnurbs* nurb )

	  nurb	Specifies the NURBS object (created with

	  Use gluBeginTrim to mark the beginning of a trimming loop,
	  and gluEndTrim to mark the end of a trimming loop. A
	  trimming loop	is a set of oriented curve segments (forming a
	  closed curve)	that define boundaries of a NURBS surface. You
	  include these	trimming loops in the definition of a NURBS
	  surface, between calls to gluBeginSurface and	gluEndSurface.

	  The definition for a NURBS surface can contain many trimming
	  loops. For example, if you wrote a definition	for a NURBS
	  surface that resembled a rectangle with a hole punched out,
	  the definition would contain two trimming loops. One loop
	  would	define the outer edge of the rectangle;	the other
	  would	define the hole	punched	out of the rectangle. The
	  definitions of each of these trimming	loops would be
	  bracketed by a gluBeginTrim/gluEndTrim pair.

	  The definition of a single closed trimming loop can consist
	  of multiple curve segments, each described as	a piecewise
	  linear curve (see gluPwlCurve) or as a single	NURBS curve
	  (see gluNurbsCurve), or as a combination of both in any
	  order. The only library calls	that can appear	in a trimming
	  loop definition (between the calls to	gluBeginTrim and
	  gluEndTrim) are gluPwlCurve and gluNurbsCurve.

	  The area of the NURBS	surface	that is	displayed is the
	  region in the	domain to the left of the trimming curve as
	  the curve parameter increases. Thus, the retained region of
	  the NURBS surface is inside a	counterclockwise trimming loop
	  and outside a	clockwise trimming loop. For the rectangle
	  mentioned earlier, the trimming loop for the outer edge of
	  the rectangle	runs counterclockwise, while the trimming loop
	  for the punched-out hole runs	clockwise.

	  If you use more than one curve to define a single trimming
	  loop,	the curve segments must	form a closed loop (that is,
	  the endpoint of each curve must be the starting point	of the
	  next curve, and the endpoint of the final curve must be the
	  starting point of the	first curve). If the endpoints of the
	  curve	are sufficiently close together	but not	exactly
	  coincident, they will	be coerced to match.  If the endpoints
	  are not sufficiently close, an error results (see

	  If a trimming	loop definition	contains multiple curves, the
	  direction of the curves must be consistent (that is, the
	  inside must be to the	left of	all of the curves). Nested
	  trimming loops are legal as long as the curve	orientations
	  alternate correctly.	If trimming curves are self-
	  intersecting,	or intersect one another, an error results.

	  If no	trimming information is	given for a NURBS surface, the
	  entire surface is drawn.

	  This code fragment defines a trimming	loop that consists of
	  one piecewise	linear curve, and two NURBS curves:

	     gluPwlCurve(..., GLU_MAP1_TRIM_2);
	     gluNurbsCurve(...,	GLU_MAP1_TRIM_2);
	     gluNurbsCurve(...,	GLU_MAP1_TRIM_3); gluEndTrim(nobj);

	  gluBeginSurface, gluNewNurbsRenderer,	gluNurbsCallback,
	  gluNurbsCurve, gluPwlCurve