I wrote
a series of five extremely short piano pieces called each
Possible World in 2005. They last between 15 and 20 seconds each, with
the exception of the slower nr. 4, which lasts around 30 seconds.
Possible World 1-4 are for one piano. Possible World nr. 5 is for one,
two, three or four pianos, with each pianist playing a different
ordering
of the same material.