You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: gpcovarf.m
    path of file: ./repos/pcm/matlab/lib/netlab3_3
    
    the code of the file until where you have to start completion: function covf = gpcovarf(net, x1, x2)
%GPCOVARF Calculate the covariance function for a Gaussian Process.
%
%	Description
%
%	COVF = GPCOVARF(NET, X1, X2) takes  a Gaussian Process data structure
%	NET together with two matrices X1 and X2 of input vectors,  and

    
function covf = gpcovarf(net, x1, x2)
%GPCOVARF Calculate the covariance function for a Gaussian Process.
%
%	Description
%
%	COVF = GPCOVARF(NET, X1, X2) takes  a Gaussian Process data structure
%	NET together with two matrices X1 and X2 of input vectors,  and
%	computes the matrix of the covariance function values COVF.
%
%	See also
%	GP, GPCOVAR, GPCOVARP, GPERR, GPGRAD
%

%	Copyright (c) Ian T Nabney (1996-2001)

errstring = consist(net, 'gp', x1);
if ~isempty(errstring);
  error(errstring);
end