Hi Ding-Dong, your quotation does not apply to my problem. The next paragraph is relevant:
There are two ways to put tremolo marks on a single note...the note should not be surrounded by braces:
However, removing the braces does not fix the problem.
[Using a single note rather than an accord (sib8 rather than <sib sol' sib>8, which would be wrong from a musical point of view) has the same wrong result.]
\repeat tremolo 7 is valid and produces a double dotted note indeed, but it is 2 triplet-eigths too short, the following note starts that amount too early. These 2 triplet-eights are exactly the missing ones between tremolo 9 and tremolo 7. Thus - I could work with \skip-s here, but will fail, when LilyPond is corrected.
Nevertheless - thanks for your help!
What I want to note: Play 9 triplet-eigths in 3 quarter-beats.
Calculation: 9 * (2/3) * (1/8) = (9*2*1) / (3*8) = 18/24 = 3/4
So, something is wrong in the LilyPond-calculation code, is it not.
By the way: It is not a strange requirement - I need it for Schubert's Erlkönig, bar 14 and others.
Radler