I understand. Let me ask you then, where should this behavior be
documented? Perhaps in the section about modifying shapes
<http://lilypond.org/doc/v2.19/Documentation/notation/modifying-shapes>
in the Notation Reference?
Currently it states:
Known issues and warnings
It is not possible to modify shapes of ties or slurs by changing
the |control-points| property if there are multiple ties or slurs
at the same musical moment – the |\tweak| command will also not
work in this case. However, the |tie-configuration| property of
|TieColumn| can be overridden to set start line and direction as
required.
Since contrary to the above this is indeed possible (as proved by the
example shared by Harm), could this paragraph be updated instead? I
think we can all agree that the documentation needs to convey accurate
information.
I was going to write a longer message but this has already gone too
out of thread. I don't know where/how this kind of issues are supposed
to be discussed. If you could point me in the right direction I'd be
happy to bring this up in the right place and in the right manner.