This indexes a token number inside a sentence. Standardly, tokens are
indexed within a sentence starting at 1 (not 0: we follow common parlance
whereby we speak of the first word of a sentence).
This is generally an individual word or feature index - it is local, and
may not be uniquely identifying without other identifiers such as sentence
and doc. However, if these are the same, the index annotation should be a
unique identifier for differentiating objects.
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.