=begin
= ToDo
* <xsl:copy-of>
* <xsl:output HTML>: <img></img> => <img/> ...
* xsl:param 
* xsl:text
* XPath functions: ruby:func(a, b)  => a.func(b) in Ruby
* <xsl:variable>xxxx</xsl:variable>, same with xsl:param
* build a output tree 

= Done
* xsl:include
* xsl:sort
* xsl:call-template
* xsl:choose
* parse_node etc.. not with @content -> as parameter
* extensions: <ruby:eval>...</ruby:eval>
              <ruby:comp ns="..." source="external" object="MyComp.new"/>
* scope of variables
* parse attributes of tags for {... } and eval this as XPath expression.
  How to disable this for specific elements?
* <xsl:for-each>

=end

