What are AST Macros?

[Steven Schveighoffer]

On Mon, 12 Jul 2010 08:12:31 -0400, bearophile <bearophileHUGS at lycos.com>  
wrote:

> Steven Schveighoffer:
>> If people agree that AST macros are superseded by mixins,
>
> String mixins are a hack, just a bit better than C preprocessor (because  
> they are scoped), they are not a replacement of clean macros.

Interesting statement, can you back it up? :)  What can you do with a  
macro that you can't do with a mixin?

-Steve