irb(main):001:0> require 'rexml/document'
=> true
irb(main):002:0> include REXML
=> Object
irb(main):003:0> d=Document.new "<foo/>"
=> <UNDEFINED> ... </>
irb(main):004:0> d.write
<foo/>=> [#<REXML::XMLDecl:0x4065e5a0 @version="1.0", @writeencoding=false, @parent=nil, @writethis=false, @standalone=nil, @encoding="UTF-8">, <foo/>]
irb(main):005:0> d.elements.add "bar"
RuntimeError: attempted adding second root element to document
        from /usr/lib/ruby/1.8/rexml/document.rb:81:in `<<'
        from /usr/lib/ruby/1.8/rexml/element.rb:893:in `add'
        from /usr/lib/ruby/1.8/rexml/child.rb:21:in `initialize'
        from /usr/lib/ruby/1.8/rexml/parent.rb:13:in `initialize'
        from /usr/lib/ruby/1.8/rexml/element.rb:53:in `initialize'
        from /usr/lib/ruby/1.8/rexml/element.rb:891:in `new'
        from /usr/lib/ruby/1.8/rexml/element.rb:891:in `add'
        from (irb):5
irb(main):006:0> d.write
<foo/><NoMethodError: undefined method `empty?' for nil:NilClass
        from /usr/lib/ruby/1.8/rexml/element.rb:663:in `write'
        from /usr/lib/ruby/1.8/rexml/document.rb:163:in `write'
        from /usr/lib/ruby/1.8/rexml/document.rb:161:in `each'
        from /usr/lib/ruby/1.8/rexml/document.rb:161:in `write'
        from (irb):6
irb(main):007:0> 


the latter exception is the problem