from-expand.rs

    let maybe_it = match expanded {
        mr_item(it) => fld.fold_item(it),
        mr_expr(_) => cx.span_fatal(pth.span,
                                    ~"expr macro in item position: "
                                    + *extname),
        mr_any(_, item_maker, _) =>
            option::chain(item_maker(), |i| {fld.fold_item(i)}),
        MacroDef(ref mdef) => {
            exts.insert((*mdef).name, (*mdef).ext);
            None
        }
    };