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_gx_fmri.m
    path of file: ./repos/spm12
    
    the code of the file until where you have to start completion: function [g,dgdx] = spm_gx_fmri(x,u,P,M)
% Simulated BOLD response to input
% FORMAT [g,dgdx] = spm_gx_fmri(x,u,P,M)
% g          - BOLD response (%)
% x          - state vector     (see spm_fx_fmri)
% P          - Parameter vector (see spm_fx_fmri)
% M          - model specification structure (see spm_nlsi)
%__________________________________________________________________________
%
% This function implements the BOLD signal model described in: 
%
% Stephan KE, Weiskopf N, Drysdale PM, Robinson PA, Friston KJ (2007)
% Comparing hemodynamic models with DCM. NeuroImage 38: 387-401.
%__________________________________________________________________________
% Copyright (C) 2008 Wellcome Trust Centre for Neuroimaging
 
% Karl Friston & Klaas Enno Stephan
% $Id: spm_gx_fmri.m 6262 2014-11-17 13:47:56Z karl $
 
 
% Biophysical constants for 1.5T
%==========================================================================
 
% time to echo (TE) (default 0.04 sec)
%--------------------------------------------------------------------------
TE  = 0.04;
 
% resting venous volume (%)
%--------------------------------------------------------------------------
V0  = 4;

% estimated region-specific ratios of intra- to extra-vascular signal 
%--------------------------------------------------------------------------
ep  = exp(P.epsilon);
 
% slope r0 of intravascular relaxation rate R_iv as a function of oxygen 
% saturation S:  R_iv = r0*[(1 - S)-(1 - S0)] (Hz)
%--------------------------------------------------------------------------
r0  = 25;
 
% frequency offset at the outer surface of magnetized vessels (Hz)
%--------------------------------------------------------------------------
nu0 = 40.3; 
 
% resting oxygen extraction fraction
%--------------------------------------------------------------------------
E0  = 0.4;
 
%-Coefficients in BOLD signal model
%==========================================================================
k1  = 4.3*nu0*E0*TE;
k2  = ep*r0*E0*TE;
k3  = 1 - ep;

    
function [g,dgdx] = spm_gx_fmri(x,u,P,M)
% Simulated BOLD response to input
% FORMAT [g,dgdx] = spm_gx_fmri(x,u,P,M)
% g          - BOLD response (%)
% x          - state vector     (see spm_fx_fmri)
% P          - Parameter vector (see spm_fx_fmri)
% M          - model specification structure (see spm_nlsi)
%__________________________________________________________________________
%
% This function implements the BOLD signal model described in: 
%
% Stephan KE, Weiskopf N, Drysdale PM, Robinson PA, Friston KJ (2007)
% Comparing hemodynamic models with DCM. NeuroImage 38: 387-401.
%__________________________________________________________________________
% Copyright (C) 2008 Wellcome Trust Centre for Neuroimaging
 
% Karl Friston & Klaas Enno Stephan
% $Id: spm_gx_fmri.m 6262 2014-11-17 13:47:56Z karl $
 
 
% Biophysical constants for 1.5T
%==========================================================================
 
% time to echo (TE) (default 0.04 sec)
%--------------------------------------------------------------------------
TE  = 0.04;
 
% resting venous volume (%)
%--------------------------------------------------------------------------
V0  = 4;

% estimated region-specific ratios of intra- to extra-vascular signal 
%--------------------------------------------------------------------------
ep  = exp(P.epsilon);
 
% slope r0 of intravascular relaxation rate R_iv as a function of oxygen 
% saturation S:  R_iv = r0*[(1 - S)-(1 - S0)] (Hz)
%--------------------------------------------------------------------------
r0  = 25;
 
% frequency offset at the outer surface of magnetized vessels (Hz)
%--------------------------------------------------------------------------
nu0 = 40.3; 
 
% resting oxygen extraction fraction
%--------------------------------------------------------------------------
E0  = 0.4;
 
%-Coefficients in BOLD signal model
%==========================================================================
k1  = 4.3*nu0*E0*TE;
k2  = ep*r0*E0*TE;
k3  = 1 - ep;
 
%-Output equation of BOLD signal model
%==========================================================================
v   = exp(x(:,4));
q   = exp(x(:,5));
g   = V0*(k1 - k1.*q + k2 - k2.*q./v + k3 - k3.*v);

if nargout == 1, return, end