I wish to thank Jeremy Allison and my parents for the inspiration; Islene Calciolari Garcia and Jorge Stolfi for the long discussions about this subject; Richard M. Stallman, for supporting the idea of writing this paper, for having reviewed it and mainly for having started the whole Free Software movement. I'd also like to thank Doug DeJulio and Jim Kingdon, and especially Paul Gallagher, Tom Tromey and Mary Lynn Kostash for the very careful reviews and the many useful suggestions to improve the paper. I claim full responsibility for any errors I may have insisted in introducing.