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: f_HH.m
    path of file: ./repos/VBA-toolbox/demos/_models
    
    the code of the file until where you have to start completion: function [fx] = f_HH(Xt, Theta, ut, inF)
% Hodgkin-Hoxley membrane potential evolution function
% function [fx] = f_HH(Xt,Theta,ut,inF)

    
function [fx] = f_HH(Xt, Theta, ut, inF)
% Hodgkin-Hoxley membrane potential evolution function
% function [fx] = f_HH(Xt,Theta,ut,inF)
% IN:
%   - Xt: system's states, ie:
%       Xt(1): membrane depolarization (mV)
%       Xt(2:4): type I-II-III ion channel opening probabilities, in
%       inverse-sigmoid space (Gaussian transformation)
%   - Theta: evolution parameters (see bellow)
%   - ut: input current
%   - inF: mandatory for field .delta_t for time discretization
% OUT:
%   - fx: the evolution function evaluated at Xt


deltat = inF.delta_t;
C = exp(Theta(1)); % neuron capacitance
gK = 36*exp(Theta(2));
gNa = 120*exp(Theta(3));
gL = 0.3*exp(Theta(4));

EK = -12;
ENa = 115;
EL = 10.6;

V = Xt(1);
m = VBA_sigmoid(Xt(2));
n = VBA_sigmoid(Xt(3));
h = VBA_sigmoid(Xt(4));

an = (0.1 - 0.01*V)./(exp(1-0.1*V)-1);
am = (2.5 - 0.1*V)./(exp(2.5-0.1*V)-1);
ah = 0.07*exp(-V./20);
bn = 0.125*exp(-V/80);
bm = 4*exp(-V/18);
bh = 1./(exp(3-0.1*V)+1);

r = 1e-2; % for numerical stability

xdot = [ (-gNa*m.^3*h*(V-ENa) - gK*n.^4*(V-EK) - gL*(V-EL) + ut)/C
        (am*(1-m) - bm*m)/(m-m.^2+r)
        (an*(1-n) - bn*n)/(n-n.^2+r)
        (ah*(1-h) - bh*h)/(h-h.^2+r)   ];

fx = Xt + deltat.*xdot;

end