Ideas on further XSLT optimization
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1. Cache same-expr instantiations with same current node. Think about how
   to determine it compile-time. e.g.
   <xsl:if test="@foo"> <blah att="{@foo}"/>

2. Optimize xsl:copy-of: do not disposeResult on selectInstChanged_, 
   merge top-level copied nodes instead. Also, make different copiers for
   top-level nodes only (to save extra 8 bytes in each  non-top-level copier).

3. In future, may generate special FO instruction nodes instead of plain grove 
   nodes.
