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: dace_corrspherical.m
    path of file: ./repos/PlatEMO/PlatEMO/Algorithms/Multi-objective optimization/MO-L2SMEA/krg/dace
    
    the code of the file until where you have to start completion: function  [r, dr] = dace_corrspherical(theta, d)
%Spherical correlation function,
%
%           n
%   r_i = prod max(0, 1 - 1.5(theta_j*d_ij) + .5(theta_j*d_ij)^3) ,  i = 1,...,m
%          j=1
%
% If length(theta) = 1, then the model is isotropic:
% all  theta_j = theta .
%
% Call:    r = dace_corrspherical(theta, d)
%          [r, dr] = dace_corrspherical(theta, d)
%
% theta :  parameters in the correlation function
% d     :  m*n matrix with differences between given data points
% r     :  correlation

    
function  [r, dr] = dace_corrspherical(theta, d)
%Spherical correlation function,
%
%           n
%   r_i = prod max(0, 1 - 1.5(theta_j*d_ij) + .5(theta_j*d_ij)^3) ,  i = 1,...,m
%          j=1
%
% If length(theta) = 1, then the model is isotropic:
% all  theta_j = theta .
%
% Call:    r = dace_corrspherical(theta, d)
%          [r, dr] = dace_corrspherical(theta, d)
%
% theta :  parameters in the correlation function
% d     :  m*n matrix with differences between given data points
% r     :  correlation
% dr    :  m*n matrix with the Jacobian of r at x. It is
%          assumed that x is given implicitly by d(i,:) = x - S(i,:),
%          where S(i,:) is the i'th design site.

% hbn@imm.dtu.dk
% Last update April 12, 2002

[m n] = size(d);  % number of differences and dimension of data
if  length(theta) == 1
    theta = repmat(theta,1,n);
elseif  length(theta) ~= n
    error(sprintf('Length of theta must be 1 or %d',n))
else
    theta = theta(:).';
end