Once a --reintegrate merge is done from branch to trunk, the branch is no longer usable for further work. It's not able to correctly absorb new trunk changes, nor can it be properly reintegrated to trunk again. For this reason, if you want to keep working on your feature branch, we recommend destroying it and then re-creating it from the trunk:
$ svn delete http://svn.example.com/repos/calc/branches/my-calc-branch \
-m "Remove my-calc-branch, reintegrated with trunk in r391."
Committed revision 392.
$ svn copy http://svn.example.com/repos/calc/trunk \
-m "Recreate my-calc-branch from trunk@HEAD."
Committed revision 393.