There still seems to be some kind of package clash. (kpsewhich shows
that the sty files are installed correctly). I even tried it with all
packages that gmeometric is based on. Now it seems to have a problem
with "itemize" and still says that "geometry is only allowed in the
preamble". Never mind. I might sort out something eventually.
In the meantime I have been exploring alternative scenarios, i.e. have
the page in landscape A4 and just print it off in A3. Sounds easy, one
might think...
example 1
\usepackage{lscape} or \usepackage{pdflscape} (they are supposedly
different, but in my case they came up exactly the same in LYX)
\begin{landscape}
*** content on a landscape page ***
\end{landscape}
This rotates very nicely the page, but the headers stay on the top (as
they should, because in the normal page you just want to turn your
document). However as I want this page to be in A3 later on the header
has to be on the long site.
example2
\usepackage{portland} should in theory turn the whole thing including
headers, but refuses to do so when I try it!
\landscape
*** content on a landscape page ***
\portrait
Well, outcome just as in "lscape", no rotation of the headers.
example 3
keep pages empty and replace them later over the command line using "pdftk".
Possibly ok if you don't have fancy headers with section titles in them
and if you have endless time to replace all single pages with the right
page numbers.
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So if anyone has an idea how to rotate the headers, it would be very
much appreciated!
Joanne