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_fmri.m
    path of file: ./repos/spm
    
    the code of the file until where you have to start completion: function [f,dfdx,D,dfdu] = spm_fx_fmri(x,u,P,M)
% State equation for a dynamic [bilinear/nonlinear/Balloon] model of fMRI
% responses
% FORMAT [f,dfdx,D,dfdu] = spm_fx_fmri(x,u,P,M)
% x      - state vector
%   x(:,1) - excitatory neuronal activity            ue
%   x(:,2) - vascular signal                          s
%   x(:,3) - rCBF                                  ln(f)
%   x(:,4) - venous volume                         ln(v)
%   x(:,5) - deoyxHb                               ln(q)
%  [x(:,6) - inhibitory neuronal activity             ui
%
% f      - dx/dt
% dfdx   - df/dx
% dfdu   - df/du
% D      - delays
%
%__________________________________________________________________________
%
% References for hemodynamic & neuronal state equations:
% 1. Buxton RB, Wong EC & Frank LR. Dynamics of blood flow and oxygenation

    
function [f,dfdx,D,dfdu] = spm_fx_fmri(x,u,P,M)
% State equation for a dynamic [bilinear/nonlinear/Balloon] model of fMRI
% responses
% FORMAT [f,dfdx,D,dfdu] = spm_fx_fmri(x,u,P,M)
% x      - state vector
%   x(:,1) - excitatory neuronal activity            ue
%   x(:,2) - vascular signal                          s
%   x(:,3) - rCBF                                  ln(f)
%   x(:,4) - venous volume                         ln(v)
%   x(:,5) - deoyxHb                               ln(q)
%  [x(:,6) - inhibitory neuronal activity             ui
%
% f      - dx/dt
% dfdx   - df/dx
% dfdu   - df/du
% D      - delays
%
%__________________________________________________________________________
%
% References for hemodynamic & neuronal state equations:
% 1. Buxton RB, Wong EC & Frank LR. Dynamics of blood flow and oxygenation
%    changes during brain activation: The Balloon model. MRM 39:855-864,
%    1998.
% 2. Friston KJ, Mechelli A, Turner R, Price CJ. Nonlinear responses in
%    fMRI: the Balloon model, Volterra kernels, and other hemodynamics.
%    Neuroimage 12:466-477, 2000.
% 3. Stephan KE, Kasper L, Harrison LM, Daunizeau J, den Ouden HE,
%    Breakspear M, Friston KJ. Nonlinear dynamic causal models for fMRI.
%    Neuroimage 42:649-662, 2008.
% 4. Marreiros AC, Kiebel SJ, Friston KJ. Dynamic causal modelling for
%    fMRI: a two-state model.
%    Neuroimage. 2008 Jan 1;39(1):269-78.
%__________________________________________________________________________

% Karl Friston & Klaas Enno Stephan
% Copyright (C) 2002-2022 Wellcome Centre for Human Neuroimaging


% options
%--------------------------------------------------------------------------
if nargin < 4, M = struct([]); end