[wg-camlp4] Changes to the parsetree

[Leo White]

> This is rather minor (especially considering that other use cases are sufficient to justify "item" attributes on
> individual types), but isn't it possible to provide a custom function for a specific type instead of asking the
> extension to generate the function?

You can, but it is difficult to do it for mutually recursive types since the
functions will probably also have to be mutually recursive.