function [g1, g2] = whitefixedXwhitefixedKernGradient(whiteKern1, whiteKern2, x1, x2, covGrad)

% WHITEFIXEDXWHITEFIXEDKERNGRADIENT Compute a cross gradient between two WHITEFIXED kernels.
%
%	Description:
%
%	[G1, G2] = WHITEFIXEDXWHITEFIXEDKERNGRADIENT(WHITEKERN1, WHITEKERN2,
%	X, COVGRAD) computes cross gradient of parameters of a cross kernel
%	between two whitefixed noise kernels for the multiple output kernel.
%	 Returns:
%	  G1 - gradient of the parameters of the first kernel, for ordering
%	   see whitefixedKernExtractParam.
%	  G2 - gradient of the parameters of the second kernel, for ordering
%	   see whitefixedKernExtractParam.
%	 Arguments:
%	  WHITEKERN1 - the kernel structure associated with the first white
%	   noise kernel.
%	  WHITEKERN2 - the kernel structure associated with the second white
%	   noise kernel.
%	  X - inputs for which kernel is to be computed.
%	  COVGRAD - gradient of the objective function with respect to the
%	   elements of the cross kernel matrix.
%
%	[G1, G2] = WHITEFIXEDXWHITEFIXEDKERNGRADIENT(WHITEKERN1, WHITEKERN2,
%	X1, X2, COVGRAD) computes cross kernel terms between two whitefixed
%	noise kernels for the multiple output kernel.
%	 Returns:
%	  G1 - gradient of the parameters of the first kernel, for ordering
%	   see whitefixedKernExtractParam.
%	  G2 - gradient of the parameters of the second kernel, for ordering
%	   see whitefixedKernExtractParam.
%	 Arguments:
%	  WHITEKERN1 - the kernel structure associated with the first white
%	   noise kernel.
%	  WHITEKERN2 - the kernel structure associated with the second white
%	   noise kernel.
%	  X1 - row inputs for which kernel is to be computed.
%	  X2 - column inputs for which kernel is to be computed.
%	  COVGRAD - gradient of the objective function with respect to the
%	   elements of the cross kernel matrix.
%	
%
%	See also
%	MULTIKERNPARAMINIT, MULTIKERNCOMPUTE, WHITEFIXEDKERNPARAMINIT, WHITEFIXEDKERNEXTRACTPARAM


%	Copyright (c) 2006 Neil D. Lawrence
% 	whitefixedXwhitefixedKernGradient.m SVN version 20
% 	last update 2008-06-06T22:08:25.000000Z

arg{1}=x1;
if nargin < 5
  covGrad = x2;
  x2 = x1;
else
  arg{2}=x2;
end
if size(x1, 2) > 1 | size(x2, 2) > 1
  error('Input can only have one column');
end
g1 = 0;
g2 = 0;