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20141128 (Friday, 28 November 2014)
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Worked on the German docs about :ref:`welfare`.

While working on :ticket:`16`, I stumbled over an relatively old
problem which I registered as :ticket:`20` and then fixed.

Checkin at 13:54 fixes :ticket:`20`.

It took me then a few hours to find the explanation for :ticket:`16`:

The problem occurs only when acting as another user.  E.g. I am Rolf
and acting as Theresia (who is not manager).  I ask to create a new
Confirmation.  A Confirmation is a UserAuthored.  When Lino creates a
UserAuthored instance, the author will be set to Rolf because this is
my *real* identity. And then Theresia is not allowed to modify other
users' work because she is not a manager.

The solution was very subtle: `get_row_permission` must be a bit more
permissive: when user A is acting as user B, then neither A nor B are
considered to be "other user".

TODO: 

- I'll probably also move the `UserAuthored` mixin to
  :mod:`lino.modlib.users`.

- Maybe also make it configurable whether on_create puts `user` or
  `subst_user`.

- Write test cases to verify these subtleties.