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: covRQ.m
    path of file: ./repos/Gaussian-Process-Regression/gpml-matlab-v4.2-2018-06-11/cov
    
    the code of the file until where you have to start completion: function [K,dK] = covRQ(mode, par, hyp, varargin)

% Rational Quadratic covariance function.
% The covariance function is parameterized as:

    
function [K,dK] = covRQ(mode, par, hyp, varargin)

% Rational Quadratic covariance function.
% The covariance function is parameterized as:
%
% k(x,z) = [1 + maha(x,z)/(2*alpha)]^(-alpha)
%
% where maha(x,z) is a Mahalanobis distance and alpha is the shape parameter
% for the RQ covariance. The hyperparameters are:
%
% hyp = [ hyp_maha
%         log(alpha) ]
%
% Copyright (c) by Carl Edward Rasmussen and Hannes Nickisch, 2016-05-23.
%
% See also COVFUNCTIONS.M.

if nargin < 1, error('Mode cannot be empty.'); end