Pál, can you please comment? It looks as if support for this
ligature form is missing...
It's not missing, but it means something else: the duration sequence
1 1 \longa.
```
\[ a'1 c'' a'\longa \]
```
Indeed, this gives the right optical result, thanks!
So long I thought that there's no need to warn about ignored tweaks
(related to white mensural ligatures), but now I'm not so sure.
I think that warnings makes sense if LilyPond decides to do something
else.
I may also add a complete description in the Notation manual.
This would be helpful, too :-)
Werner