>Suppose you have an XML format, <library> that has books and authors[1]. The elements look something like this: <book author-id="0000"/>, <author id="0000" name="Plath"/>. Obviously, you want to make sure that book's author-id attribute will always refer to an author that actually exists. This is something a (Java-style) static type system can't do: it doesn't know at compile-time what the contents of a variable will be. But spec can do this because it is a runtime check[2]: you'd just write a function that ensures all author-ids refer to extant authors and register it with spec, telling it that this must be true for valid <library>s.

So is it similar to or different from assertions?