I didn't have the time to test the changes, but it seems to me that making append_branch depend on the _mode=0, breaks what the lookup_to_dset function had been thought for, adding new branches to the dset so that new branches can be added to a transaction via t_append_branches (tsilo relies on those two functions to work). Maybe I don't understand what are you trying to achieve calling lookup_to_dset with a ruri, but what's the purpose of calling a lookup without rewriting the r-uri nor appending new branches?

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