abstract Book

title -> string

subtitle -> string ?// subtitle does not have to be a child of the title because title is

// mandatory, although conceptually subtitle is a point about title as well

page -> integer// if we don't really care about individual pages, just the number

[ page >= 2 ]

or format// the choice between xor and or depends on the context. For example,

paper// whether the book store has a single entry for a book with multiple

hardcover ?// formats or whether there must be a separate entry with exactly one

electronic// format at the time

kind -> Kind// can be modeled as an enumeration

GenerativeProgramming : Book

[ title = "Generative Programming" ]

[ page = 589 ]

[ paper ]

[ electronic ]// because the type of page is integer

[ no hardcover ]// the book is in both paper and electronic formats but not hardcover