  Mathematical model theory carries a heavy load of notation, and HTML is not the best container for it. In what follows, syntactic objects (languages, theories, sentences) are generally written in roman or greek letters (for example L, T, φ), and set-theoretic objects such as structures and their elements are written in italic (A, a). Two exceptions are that variables are italic (x, y) and that sequences of elements are written with lower case roman letters (a, b). 