The first draft of these notes was born in the winter of 2002. At that time, I was a visiting scholar at the University of Waterloo. Originally, those notes were not intended as a book, but as a quick reference for not forgetting the models I was implementing for my research. After eight years, I am with Universidad de Castilla-La Mancha. During these years, the notes have been growing up little by little, ceaselessly. During the summer of 2009, I have reorganized the notes in the present book.
