Additional Exercise 2 for Session 11.

The GEE estimating equation is given in many different forms, e.g. (9.7) in
Davis (2002), (8.27) and (8.31) in McCulloch & Searle (2000), (8.2.4) in Diggle
et al. (2002). Compare these equations and show they are in fact the same. Also,
derive or justify the equation when an independence working correlation is
assumed.