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: gpr.m
    path of file: ./repos/chebfun
    
    the code of the file until where you have to start completion: function varargout = gpr(x, y, varargin)
%GPR    Gaussian process regression
%
%   F = GPR(X, Y) returns a CHEBFUN F defined on [min(X),max(X)]
%   representing the posterior mean of a Gaussian process with prior mean 0
%   and squared exponential kernel
%               k(x,x')
    
function expansion of the mean
    alpha = L'\(L\y);

    % constuct a Chebfun approximation for the posterior distribution mean
    if opts.trig && ~opts.sigmaY
        f = chebfun(@(z) mean(alpha, x, z, opts), opts.dom, 'trig', ...
            'eps', 1e-12,'splitting','on');
    else
        f = chebfun(@(z) mean(alpha, x, z, opts), opts.dom, ...
            'eps', 1e-12,'splitting','on');
    end