How to stop SAX parse on libxml-ruby

Stop SAX parse

Does anybody know some stop method?

require 'rubygems'
require 'libxml'
include LibXML

class PostCallbacks
 include XML::SaxParser::Callbacks

 def on_start_element(element, attributes)
   if /usr/ =~ element
     p element
     raise 'stop!'
   end
 end


end

parser = XML::SaxParser.file('test.xml')
parser.callbacks = PostCallbacks.new
begin
parser.parse
rescue
p 'hard stop'
end

It will return 1st ＜usr＞ then stop.