When tracking third party source, it is useful to have it on separate branch and merge onto trunk as needed.
This doesn't work in Fossil:
<pre>
rm -rf
mkdir
cd
fossil new bug.fossil
mkdir bug
cd bug
echo "vendor version" > test
fossil open ../bug.fossil
fossil add test
fossil ci --branch vendor -m "Import vendor source."
fossil close
rm -rf *
fossil open ../bug.fossil trunk
echo === Attemt the merge... ===
fossil merge vendor
</pre>

The script output here is:
<pre>
project-id:
server-id:
admin-user: asau (initial password is "485164")
ADDED  test
New_Version:
project-name: <unnamed>
repository:
local-root:
project-code:
server-code:
checkout:     2010-10-14 13:54:22 UTC
child:        2010-10-14 13:54:23 UTC
tags:         trunk
comment:      initial empty check-in (user: asau)
=== Attemt the merge... ===
fossil: lack both primary and secondary files
</pre>