Notes on extensibility

The choice of parameters seems somewhat ad-hoc, but is presumably based on some foreseen need for extensibility.

• Identifiers are not a parameter in the types, and they are not manipulated by the general map, acc & seq functions.
• The do syntax is not a parameter in the types, so e.g. the O'Haskell extensions of the do notation can not be accomodated in an elegant way.
• The type for the do syntax, HsStmt, is recursive...