[vtkusers] How to get vtkFiniteDifferenceGradientEstimator?

[Wolfram H Volpi]

Please show me the correct syntax for this problem.
I am trying to get  GradientEstimator from inside
vtkVolumeRayCastIsosurfaceFunctionCubic::SpecificFunctionInitialize().
vtkVolumeRayCastMapper has a GetGradientEstimator() method.
Below is what I attempted and the resulting compile error.

I would also apreciate it if you could answer these two related questions:
Where is "vtkNotUsed()" defined? 
Where is
vtkVolumeRayCastIsosurfaceFunctionCubic::SpecificFunctionInitialize called
from?

Thank you for your help,
Wolfv.

void vtkVolumeRayCastIsosurfaceFunctionCubic::SpecificFunctionInitialize(
	vtkRenderer *vtkNotUsed(ren), vtkVolume *vol, 
	VTKRayCastVolumeInfo *volumeInfo, vtkVolumeRayCastMapper *mapper )
	//originally arguement was: vtkVolumeRayCastMapper *vtkNotUsed(mapper) )
{
	vtkFiniteDifferenceGradientEstimator   *estimator;
2500	estimator = mapper->GetGradientEstimator;	
.
.
.
Compile error:
D:\vtk31Lightning\vtk\graphics\vtkVolumeRayCastIsosurfaceFunctionCubic.cxx(2500) : error
C2027: use of undefined type 'vtkVolumeRayCastMapper'

D:\vtk31Lightning\vtk\graphics\vtkVolumeRayCastFunction.h(64) : see
declaration of 'vtkVolumeRayCastMapper'
D:\vtk31Lightning\vtk\graphics\vtkVolumeRayCastIsosurfaceFunctionCubic.cxx(2500) : error
C2227: left of '->GetGradientEstimator' must point to class/struct/union