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: unitSphere.m
    path of file: ./repos/gkaModelEye/model/quadric/primitives/+quadric
    
    the code of the file until where you have to start completion: function S = unitSphere
% Returns a quadric surface matrix that defines a unit sphere
%
% Syntax:
%  S = quadric.unitSphere
%
% Description:
%   Returns a quadric surface matrix that is a sphere with semi-radii of 1
%   unit.
%
% Inputs:
%   none

    
function S = unitSphere
% Returns a quadric surface matrix that defines a unit sphere
%
% Syntax:
%  S = quadric.unitSphere
%
% Description:
%   Returns a quadric surface matrix that is a sphere with semi-radii of 1
%   unit.
%
% Inputs:
%   none
%
% Outputs:
%   S                     - 4x4 matrix of the quadric surface.
%

S = eye(4);
S(4,4)=-1;

end