Emms, Martin and Luz, Saturnino (2007).
  Machine Learning for Natural Language Processing.
  European Summer School of Logic, Language and Information,
    course reader.
  https://www.scss.tcd.ie/~luzs/publications/mlfornlp.pdf
Lewis, David (1997) Reuters-21578 Text Categorization
    Collection Distribution 1.0.
  http://kdd.ics.uci.edu/databases/reuters21578/reuters21578.html
Luz, Saturnino XML-encoded version of Reuters-21578.
  https://www.scss.tcd.ie/~luzs/t/cs4ll4/sw/reuters21578-xml/