All authors must admit to a dependence on the work of others. We are no exception. We owe a huge debt of gratitude to all the publications and reference sources that we have consulted and adapted for educational purposes. It would also have been quite impossible to have produced the material on this website without adapting a variety of authentic resources that we have regularly referred to. While every effort has been made, it has not always been possible to identify and cite the sources of all the material used or to trace all copyright holders. We will be happy to omit any contents or include any appropriate acknowledgements when they are brought to our notice.