The documentation for the sequencer loop says:
LOOP [name ,] n ... ENDLOOP To execute a loop n times. For infinite loops, "infinite"
can be specified as n. Optionally, the loop variable running from 0...(n-1) can be accessed
inside the loop via $name.
In fact the loop variable runs from 1...n, as can be seen by running this exciting
1 COMMENT "Figuring out MSL"
3 LOOP n,4
4 MESSAGE $n,1