+
+ A beam is concave when the middle notes get closer to the
+ beam than the left and right edge notes.
+
+ This is determined in two ways: by looking at the positions of the
+ middle notes, or by looking at the deviation of the inside notes
+ compared to the line connecting first and last.
+
+ The tricky thing is what to do with beams with chords. There are no
+ real guidelines in this case.