Evoral::Beats::operator>() rounds to (1.0 / PPQN), hardcoded 1/1920.0.
If the time difference between two events is smaller than 1/PPQN,
Beats::operator>() and Beats::operator<() produce ambiguous results.
The same pair of values is both "less than" and "greater than" depending
which operator is used.
While it's fine for some cases to ignore the order of nearly concurent
events, the std::priority_queue must be strictly ordered.
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