method
Math.posmod
Math.posmod(a, m) -> int
Returns a modulo m, but always in the range 0..m-1 — unlike the % operator, which keeps the sign of a (so -1 % 5 is -1, while Math.posmod(-1, 5) is 4). It is what you want for wrapping indices and grid coordinates that can go negative.
Example
program Demo {
handler Step phase Update {
let col = Math.posmod(x, grid_width) # never negative
}
}