The correct algorithm is to traverse the type-specific list of points,
find the point (if any) whose time matches that of the argument (because
we do not allow multiple points of the same type at the same time), and
then use that discovered point from the _points list.
This approach is required because the actual argument may no longer be
in the tempo map (due to a change made by another thread). The lack of
sync, however, needs investigation.
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