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But suppose that the 4 panel units in file1 do _not_ correspond to the first 4 units in file2. This is something that can easily happen in real life. So the append operation doesn't get the matching right.
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I'm curious what your reactions and answers will be, thanks

Update: off-list it was discussed that the behavior of append is indeed problematic at least when the number of units (groups) in the two datasets differ, because it is not obvious whether the missing groups in the smaller dataset would be at the beginning, at the end, or wherever. So this will probably be tackled, eventually.

cheers

sven