Assertion !is_empty() failed - yet again

[Andrew Bernard]

Lilypond 2.19.83. Yet again I see this error:

Drawing systems...
Finding the ideal number of pages...
Fitting music on 15 or 16 pages...
Drawing systems...lilypond:
/home/gub/NewGub/gub/target/linux-64/src/lilypond-git.sv.gnu.org--lilypond.git-stable-test/flower/include/interval.hh:227:
T Interval_t<T>::center() const [with T = double]: Assertion `!is_empty ()'
failed.
Aborted (core dumped)
make: *** [Makefile:57: sq1-215.pdf] Error 134

After only about 30 pages of dense string quartet music, now happens when I
just add one more note. This came up for me before with tuplets, but now a
plain note triggers this.

How do we move forward on solving this? Once more, irritatingly for me and
for all, I don;t know how to make an MWE to stimulate this.

Perhaps resort to gdb backtrace again?

Andrew