Introduction to symmetric models and the approximation lemma - update

Click here for a slightly updated version of the tutorial "Introduction to symmetric models and the approximation lemma". I have added some more history on the symmetric model technique, with bibliography, taken mainly from Jech's latest 'Set Theory'. Also I renamed 'usable symmetry generators' to 'projectable symmetry generators', to avoid sentences of the form '...we'll use the usable...' and because the name 'projectable' is a more appropriate name. Finally I tidied up the 'approximation' term overload of pages 6-7.