Some questions on properties

Hi,

I'm rather new to RDF.  Trying to interpret RDF in Prolog and
struggling with the definition I have troubles with two aspects
of properties.

I refer to the rule 6.12 fromREC-rdf-syntax-19990105:


 [6.12] propertyElt    ::= '<' propName idAttr? '>' value '</' propName '>'
                         | '<' propName idAttr? parseLiteral '>'
                               literal '</' propName '>'
                         | '<' propName idAttr? parseResource '>'
                               propertyElt* '</' propName '>'
                         | '<' propName idRefAttr? bagIdAttr? propAttr* '/>'

First of all, I cannot find what it means to give a property an ID? 
Does somebody has an example?

Also, I have little clue on the meaning of the various possibilities
implied by the last rule.  It suggests I can write

<?xml version='1.0'?>

<rdf:RDF xmlns:rdf="http://w3.org/TR/1999/PR-rdf-syntax-19990105#">
  <rdf:Description rdf:ID="t1">
    <a1 a2="hello" a3="world"/>    
  </rdf:Description>
</rdf:RDF>

The online parser at http://www.pro-solutions.com/rdfdemo/rdf.cgi
suggests:

resource('genid1') predicate('a2') literal('hello')
resource('genid1') predicate('a3') literal('world')
resource('genid0') predicate('a1') literal('')
resource('genid0') predicate('http://w3.org/TR/1999/PR-rdf-syntax-19990105#ID')
			literal('t1')
resource('genid0') predicate('http://www.w3.org/1999/02/22-rdf-syntax-ns#type')
			resource('http://w3.org/TR/1999/PR-rdf-syntax-19990105#Description')

This doesn't really make much sense to me!?

	Please help!

		--- Jan