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: spm_fx_nmda.m
    path of file: ./repos/bspm/thirdparty/spm12/toolbox/dcm_meeg
    
    the code of the file until where you have to start completion: function [f,J,Q] = spm_fx_nmda(x,u,P,M)
% state equations for neural-mass and mean-field models
% FORMAT [f,J,Q] = spm_fx_nmda(x,u,P,M)
%
% x - states and covariances
%
% x{1}(i,j,k)   - k-th state of j-th population of i-th source
%                 i.e., running over sources, pop. and states
% x{2}(:,:,i,j) - covariance among k states
%                 i.e., running ove
    
function
%==========================================================================
if mfm
    
    % covariance among states (mV^2)
    %----------------------------------------------------------------------    
    for i = 1:ns
        for j = 1:np
            Cx{i,j} = x{2}(:,:,i,j);
            Vx(i,j) = Cx{i,j}(1,1);       % population variance
        end
    end

    D   = sparse(diag([1/8 1 1]));        % diffusion
    D   = exp(P.S)*D;
    
else
    
    % neural-mass approximation to covariance of states
    %----------------------------------------------------------------------
    try
        Cx = M.Cx;
    catch
        Cx = [75    0.2    0.8;
              0.2   0.004  0  ;
              0.8   0      0.02];
    end