[1] http://www.gnu.org/licenses/gpl.html

[2] http://archives.cnn.com/2000/TECH/computing/03/28/cyberpatrol.mirrors/

[3] Open Source: the Unauthorized White Papers, Donald K. Rosenberg, IDG Books,
    2000. Quotes are from page 114, ``Effects of the GNU GPL''.

[4] In the "What License to Use?" section of
    http://www.oreilly.com/catalog/opensources/book/brian.html

This whitepaper is a condensation of an original work available at
http://alumni.cse.ucsc.edu/~brucem/open_source_license.htm