XML:
====
  - I/O avoid copies when parsing mmap'ed files, need to use
    unmutable buffers except for the last 4K to make it 0 terminated
    (or get rid of 0 termination check in the whole parser, hard !)
  - recheck > 4Gb instances for parsing, and RNG validation.
    

XSLT:
=====
  - global variable access
     for each expr in stylesheet : allocate index
     for each transformation context
       create an index table
         cache values in the transformation
	   - context evaluation
	     - cache
	     - constants at the XPAth level
	   - provide more info on compiled expressions
	     - dependances w.r.t. variables (which ones)
	     - dependency on the current node
	     - dependency on the current document
  - do not use _private but other fields in the node structures.