Override mode for local layout / how to unset parameter

[Lorenzo Bertini]

Hi,

I hope you all had wonderful holidays. I'm doing some tests with local 
layouts, and I was wondering what is the modality with which a local 
layout overrides the default: does it completely erase the latter and 
uses your edits, or does it override only the attributes you've modified?

For example:

default      |   local      |   case 1   |   case 2   |
------------------------------------------------------|
Style Test   | Style Test   |            |            |
   Attr1 foo  |   Attr1 woo  | Attr1 woo  | Attr1 woo  |
   Attr2 bar  |              |            | Attr2 bar  |
End          | End          |            |            |

Also, suppose I want to "unset" a layout attribute that is set to 
something in the default layout, for example "DocbookWrapperTag" because 
I don't want wrapping: I can obtain what I want doing
 > DocbookWrapperTag ""
but is there a better way?

Cheers

-- 
Lorenzo