The exact original surface form of a token. This is created in the
invertible PTBTokenizer. The tokenizer may normalize the token form to
match what appears in the PTB, but this key will hold the original characters.
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.