3.4 Modeling with relations  (Page 10/2)

Note that the $\mathrm{nhbr}$ relation can actually represent an arbitrarily weird board, such aslocations that look adjacent on the map but actually aren't, boards which wrap around a cylinder or toroid , or a location with a tunnel connecting it toa location far across the board (like the secret passages in the game Clue, or the harrowing sub trip through Naboo in Star Wars: The Phantom Menace .) One-way passages can be encoded as well(meaning the $\mathrm{nhbr}$ relation need not be symmetric). Actually, any graph can be represented!

A quick digression on a philosophical nuance: the domain for the above problem is not vegetables; it's types-of-vegetables.That is, we talk about whether or not carrots are yummy; this is different than talking the yumminess ofthe carrot I dropped under the couch yesterday, or the carrot underneath the chocolate sauce.In computer science, this often manifests itself as the difference between values, and types of values.As examples, we distinguish between 3 and the set of all integers, and we distinguish between particular carrots and the abstract idea of carrots.(Some languages even include types as values.) Philosophers enjoy debating how particular instances definethe abstract generalization, but for our purposes we'll take each both vegetables andtypes-of-vegetables as given.