DOCBOOK: ELEMENT declaration in DTD

[Mike Sangrey <mike@sojurn.lns.pa.us>]

I'm trying to understand DTDs better and am using DocBook to help.  Would 
someone be so kind and answer the following (probably extremely simple) 
question.

Everywhere in the DocBook DTD files one sees lines like:

   <!ELEMENT foobar - - ((some|stuff)*)>
or
   <!ELEMENT foobar - O ((some|stuff)*)>

What are the '-' and 'O' doing in there?  A case of virtual beer if you 
include an XML Spec. section citation.  :-)

Thanks loudly and with cymbals.
-- 
Mike Sangrey
mike@sojurn.lns.pa.us
Landisburg, Pa.
       There is no 'do' in faith, everywhere present within it is 'done'.