XMLdiff shows the differences between two similar XML files in the same
way 'diff' does with text files. It can also be used as a library or as
a command line tool and can work either with XML files or DOM trees.
The implementation is based on "Change detection in hierarchically
structured information", by S. Chawathe, A. Rajaraman, H.
Garcia-Molina, and J. Widom, Stanford University, 1996.