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: r2scanx.m
    path of file: ./repos/M-SPARC/src/xc/mgga
    
    the code of the file until where you have to start completion: function [ex,vx,v2x,v3x] = r2scanx(rho,sigma,tau)
% @file    r2scanx.m

    
function [ex,vx,v2x,v3x] = r2scanx(rho,sigma,tau)
% @file    r2scanx.m
% @brief   This file contains the functions computing the exchange energy density \epsilon_x and the potential V
% @authors Boqin Zhang <bzhang376@gatech.edu>
%          Phanish Suryanarayana <phanish.suryanarayana@ce.gatech.edu>
% Reference:
% Furness, James W., Aaron D. Kaplan, Jinliang Ning, John P. Perdew, and Jianwei Sun. 
% "Accurate and numerically efficient r2SCAN meta-generalized gradient approximation." 
% The journal of physical chemistry letters 11, no. 19 (2020): 8208-8215.
% Physical review letters 115, no. 3 (2015): 036402.
% Copyright (c) 2020 Material Physics & Mechanics Group, Georgia Tech.
% ==============================================================================================
    normDrho = sigma.^0.5;
    [p, alpha, DpDn, DpDDn, DalphaDn, DalphaDDn, DalphaDtau] = ex_basicr2SCANvariables(rho, normDrho, tau);
    [ex, vx, v2x, v3x] = exchanger2SCAN(rho, p, alpha, DpDn, DpDDn, DalphaDn, DalphaDDn, DalphaDtau);
    v2x = v2x ./ normDrho;  % D(rho*Ex)/D(|grad rho|) / |grad rho|
end