This stores the entire set of coreference links for one
document. Each link is stored as a pair of pointers (source and
destination), where each pointer stores a sentence offset and a
token offset. All offsets start at 0.
Returns the type associated with this annotation. This method must
return the same class type as its value type parameter. It feels like
one should be able to get away without this method, but because Java
erases the generic type signature, that info disappears at runtime.