- This patch implements deriving clauses for data instance declarations
  (toplevel and associated)
- Doesn't support standalone deriving.  This could be easily supported,
  but requires an extension of the syntax of standalone deriving clauses.
  Björn, fancy adding this?
- We cannot derive Typeable.  This seems a problem of notation, more than 
  anything else.  Why?  For a binary vanilla data type "T a b", we would 
  generate an instance Typeable2 T; ie, the instance is for the constructor
  alone.  In the case of a family instance, such as (S [a] (Maybe b)), we
  simply have no means to denote the associated constuctor.  It appears to
  require type level lambda - something like (/\a b. S [a] (Maybe b).
- Derivings are for *individual* family *instances*, not for entire families.
  Currently, I know of no simple translation of class instances for entire 
  families to System F_C.  This actually seems to be similar to implementing
  open data types à la Löh & Hinze.
- This patch only covers data types, not newtypes.