Frank Mittelbach wrote:
> I haven't been through with reading all the discussions on xparse, but before
> I start commenting on one or the other aspect that got discussed I want
> briefly point out some points on the history of xparse and the thoughts we (or
> I?) had when it was originally written.
> As a short summary:
>  layer -1 = user interface definition for layer 0
>  layer 0  = DTD document type definition (no layout aspects)
>  layer 1  = layout/design definition for given DTD from layer 0
>  layer 2  = predefined, customizable templates for use in layer 1 (applicable
>             to more than one DTD)
>  layer 3  = programming constructs to build objects of layer 2

De-lurking to clarify a point:

In other words, an author will only (yeah, right) use layer -1, while
the designer who customizes the layout for the book series will use layer 0.

Who uses layer 1?  I’d guess it’ll be the guy who decides how to
logically divide the book—into Parts, Chapters, &c.  Is that right?

Then who uses layers 2 & 3?

—Joel Salomon