This commit contains known bugs.

The first two spans in the assisted performance
of Song Six need to be debugged carefully, step
by step.
I'm trying to make repeating winds work properly:
1. Hold down a key for the first span until the winds
repeat.
2. Debug what happens when the key is released:
The second (rest) span should play for its normal
duration (ca. 2 sec.) but the cursor goes straight to
the beginning of the third span, without waiting...