Aloha all,

The attached patch initializes the existing variable shortn, adds it to
two caption situations in addition to the one already coded, and adds
some description to the manual.

With this patch, #+CAPTION: [Short caption]{Long caption.} in the
Org-mode source exports to \caption[Short caption]{Long caption.} in the
LaTeX export, which is, I think, as it should be.

Many thanks to Nick Dokos who made me understand why earlier patches
hadn't shown up on the patchwork server (wrong mime type, evil mail
client).  This one sent with gnus and fingers crossed.

All the best,
Tom