mod
Mod
x!y
The Mod, Modulo, Remainder or Residue[1] function, denoted by !, is used for modulo in K.
Up until the K4 dialect, the modulo operator was left atomic on account of the rotate overload.
From K5 onward, the modulo primitive is right atomic.[2]
3!12 0 3!11 2 5!15 0 3!1 2 3 4 5 6 7 8 9 10 11 12 1 2 0 1 2 0 1 2 0 1 2 0