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: stumpff.m
    path of file: ./repos/Orbital_Mechanics
    
    the code of the file until where you have to start completion: function [Cz, Sz] = stumpff(z)
    %% Stumpff Functions
    %
    % Jeremy Penn
    % 22 September 2017

    
function [Cz, Sz] = stumpff(z)
    %% Stumpff Functions
    %
    % Jeremy Penn
    % 22 September 2017
    %
    % Revision:19/10/17
    %
    % function [Cz, Sz] = Stumpff(z)
    %
    % Purpose:  This function calculates the given Stumpff functions.
    % 
    % Inputs:   o z - The argument of the Stumpff function.
    %
    % Outputs:  o Cz - The cosine Stumpff function.
    %           o Sz - The sine Stumpff function.
    %

     if (z > 0)
         Sz = (z^0.5 -sin(z^0.5))/z^1.5;    %C3 Stumpff
         Cz = (1 - cos(z^0.5))/z;           %C2 Stumpff
     elseif (z < 0)
         Sz = (sinh(-z^0.5) - (-z)^0.5)/((-z)^1.5); %C3 Stumpff (negative argument)
         Cz = (cosh(-z^0.5) - 1)/(-z);              %C2 Stumpff (negative argument)
     else
         Sz = 1/6;
         Cz = 0.5;
     end