Exclusive prefix sum
A prefix sum is a scan where the operator is addition. An exclusive prefix sum is likewise an exclusive scan where the operator is addition, but because addition has an inverse, an even more efficient implementation is possible:
def expresum [n] (xs: [n]i64) : *[n]i64 =
map2 (-) (scan (+) 0 xs) xs> expresum [1,2,3,4][0i64, 1i64, 3i64, 6i64]