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_erp.m
    path of file: ./repos/spm12/toolbox/dcm_meeg
    
    the code of the file until where you have to start completion: function [y] = spm_gx_erp(x,u,P,M)
% observer for a neural mass model of event related potentials
% FORMAT [y] = spm_gx_erp(x,u,P,M)
% x      - state vector
% y      - measured voltage y = L*x(:)
%__________________________________________________________________________
%
% David O, Friston KJ (2003) A neural mass model for MEG/EEG: coupling and
% neuronal dynamics. NeuroImage 20: 1743-1755
%__________________________________________________________________________
% Copyright (C) 2008 Wellcome Trust Centre for Neuroimaging
 
% Karl Friston
% $Id: spm_gx_erp.m 6112 2014-07-21 09:39:53Z karl $

    
function [y] = spm_gx_erp(x,u,P,M)
% observer for a neural mass model of event related potentials
% FORMAT [y] = spm_gx_erp(x,u,P,M)
% x      - state vector
% y      - measured voltage y = L*x(:)
%__________________________________________________________________________
%
% David O, Friston KJ (2003) A neural mass model for MEG/EEG: coupling and
% neuronal dynamics. NeuroImage 20: 1743-1755
%__________________________________________________________________________
% Copyright (C) 2008 Wellcome Trust Centre for Neuroimaging
 
% Karl Friston
% $Id: spm_gx_erp.m 6112 2014-07-21 09:39:53Z karl $

% parameterised lead field times [perturbations] of states
%--------------------------------------------------------------------------
L  = spm_lx_erp(P,M.dipfit);

% project onto spatial modes
%--------------------------------------------------------------------------
if isfield(M,'U')
    L = M.U'*L;
end