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_ssm2mtf.m
    path of file: ./repos/spm/toolbox/spectral
    
    the code of the file until where you have to start completion: function [mtf,Hz] = spm_ssm2mtf(dfdx,dfdu,dgdx,Hz)
% computes cross spectral density from state space representation

    
function [mtf,Hz] = spm_ssm2mtf(dfdx,dfdu,dgdx,Hz)
% computes cross spectral density from state space representation
% FORMAT [mtf,Hz] = spm_ssm2mtf(dfdx,dfdu,dgdx,Hz)
%
% dfdx - Jacobian
% dfdu - input matrix  [default: 1]
% dgdx - output matrix [default: 1]
% Hz   - frequencies   [default: based on maximum eigenvalue]
%
% mtf  - directed or modulation transfer function
%__________________________________________________________________________

% Karl Friston
% Copyright (C) 2008-2022 Wellcome Centre for Human Neuroimaging


% preliminaries
%--------------------------------------------------------------------------
if nargin < 2, dfdu = [];      end