As of Fall 2008, "eigen-two" is dual-licensed.  Specifically, develop-
ers can choose between two licenses:

      LGPL3: GNU Lesser General Public License, version 3 (June 2007)
or
      GPL3:  GNU General Public License, version 3 (June 2007)

Based on an "eigen-two" wiki page that existed as of  Fall 2008, you'd
use LGPL3 in most cases, because it's less restrictive. However, you'd
use GPL3 in cases that require it for legal reasons.  For example,  if
you link a GPL2-licensed program against "eigen-two", GPL3 would apply
to "eigen-two".