glGetClipPlane - return the coefficients of the specified
clipping plane

glGetClipPlane
	glGetClipPlane(plane) -> equation

void glGetClipPlane( GLenum plane,
                     GLdouble	*equation )

plane	    Specifies a	clipping plane.	 The number of
          clipping planes depends on the implementation, but
          at least six clipping planes are supported.	 They
          are	identified by symbolic names of	the form
          GL_CLIP_PLANEi where 0 < i < GL_MAX_CLIP_PLANES.

equation  Returns four double-precision values that are the
          coefficients of the	plane equation of plane	in eye
          coordinates.  The initial value is (0, 0, 0, 0).

glGetClipPlane returns in equation the four coefficients of
the plane equation for plane.

It is	always the case	that GL_CLIP_PLANEi = GL_CLIP_PLANE0 +
i.

If an	error is generated, no change is made to the contents
of equation.

GL_INVALID_ENUM is generated if plane	is not an accepted
value.

GL_INVALID_OPERATION is generated if glGetClipPlane is
executed between the execution of glBegin and	the
corresponding	execution of glEnd.

glClipPlane