Oldak Quill wrote:
Still thinking about our hypothetical GFDL and
CC-by-sa dual-licensed
document, even if the CC-by-sa license is used to "invoke" the right
to a legitimate derivative work, this work will still have derived
from a GFDL-licensed document and would also have to be licensed under
the GFDL. At least, this is my impression of licensing and I may be
wrong.
You can actually do both. You can license under GFDL *OR* CC-BY-SA, or
under GFDL *AND* CC-BY-SA (the latter provided that the licenses are
compatible, which may or may not be the case). In the first case a
derivative work may be under either of the licenses, or again under both
of them. In the second, it would have to be under both.