pausable explanation for the Document Series

[Keith Moore]

> For protocol work, we should
> have a tight scope on what is contained in the draft (& later PS).

tight scope is good, but there's a limit.  We tried to keep tight scope in
MIME, and in so doing, we failed to recognize some of MIME's limitations until
it was too late to change it.  I think it's useful to limit the scope of a
core protocol spec and to make it extensible, but it's also useful to 
at least map out how you expect likely extensions to work, and even to 
try a few of them - just don't put them into the base spec.