names=system
visibility=public
kind=defined

--- system -> String | nil
DTD のシステム識別子を返します。

DTD が外部サブセットを含んでいない場合は nil を返します。

  require 'rexml/document'
  doctype = REXML::Document.new(<<EOS).doctype
  <!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" 
           "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">
  EOS
  doctype.system # => "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"
  doctype.public  # => "-//W3C//DTD XHTML 1.0 Strict//EN"
  
  doctype = REXML::Document.new(<<EOS).doctype
  <!DOCTYPE root SYSTEM "foobar">
  EOS
  doctype.system # => "foobar"
  doctype.public  # => nil

