In the doc, we can read
`play 47` actually means play the 47th note on the piano.
well this is not true. A0 is the first note on a 88 keys piano right ?
play :a0 sleep 1 play : 21 # first note on a 88 keys piano sleep 1
In fact, the 47 midi note value is corresponding to the 27th key of a 88 keys piano