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_hdm.m
    path of file: ./repos/ExploreASL/External/SPMmodified
    
    the code of the file until where you have to start completion: function [y] = spm_gx_hdm(x,u,P,M)
% Simulated BOLD response to input.
% FORMAT [y] = spm_gx_hdm(x,u,P,M)
% y    - BOLD response (%)
% x    - state vector     (see spm_fx_fmri)
% P    - Parameter vector (see spm_fx_fmri)
%__________________________________________________________________________
%
% 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.
%__________________________________________________________________________

    
function [y] = spm_gx_hdm(x,u,P,M)
% Simulated BOLD response to input.
% FORMAT [y] = spm_gx_hdm(x,u,P,M)
% y    - BOLD response (%)
% x    - state vector     (see spm_fx_fmri)
% P    - Parameter vector (see spm_fx_fmri)
%__________________________________________________________________________
%
% 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_hdm.m 6856 2016-08-10 17:55:05Z karl $


% biophysical constants for 1.5 T: 
%==========================================================================

% hemodynamic parameters
%--------------------------------------------------------------------------
%   H(1) - signal decay                                   d(ds/dt)/ds)
%   H(2) - autoregulation                                 d(ds/dt)/df)
%   H(3) - transit time                                   (t0)
%   H(4) - exponent for Fout(v)                           (alpha)
%   H(5) - resting oxygen extraction                      (E0)
%   H(6) - ratio of intra- to extra-vascular components   (epsilon)
%--------------------------------------------------------------------------
if isstruct(P)
    H     = [0.64 0.32 2.00 0.32 0.4];
    for i = 1:numel(P.decay)
        H(6)   = P.epsilon;
        y(i,1) = spm_gx_hdm(x(i,:),u(i),H,M);
    end