books.google.co.uk - A textbook on modal logic, intended for readers already acquainted with the elements of formal logic, containing nearly 500 exercises. Brian F. Chellas provides a systematic introduction to the principal ideas and results in contemporary treatments of modality, including theorems on completeness and...http://books.google.co.uk/books/about/Modal_Logic.html?id=YupiXWV5j6cC&q=closed+under+intersections&utm_source=gb-gplus-shareModal Logic
