The standard key for storing a projected category in the map, as a String.
For any word (leaf node), the projected category is the syntactic category
of the maximal constituent headed by the word. Used in SemanticGraph.
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.