require 'xml/xxpath'

d=REXML::Document.new <<EOS
  <foo>
    <bar x="hello">
      <first>
        <second>pingpong</second>
      </first>
    </bar>
    <bar x="goodbye"/>
  </foo>
EOS

XML::XXPath.new("/foo/bar").all(d)
=> [<bar x='hello'> ... </>, <bar x='goodbye'/>]

XML::XXPath.new("/bar").all(d)
=> []

XML::XXPath.new("/foo/bar").all(d.root)
=> []

XML::XXPath.new("/bar").all(d.root)
=> [<bar x='hello'> ... </>, <bar x='goodbye'/>]


firstelt = XML::XXPath.new("/foo/bar/first").first(d)
=> <first> ... </>

XML::XXPath.new("/first/second").all(firstelt)
=> []

XML::XXPath.new("/second").all(firstelt)
=> [<second> ... </>]