# addMod(_:modulus:) Returns `(self + other) % modulus`. ``` func addMod(_ other: UInt256, modulus: UInt256) -> UInt256 ``` ## Parameters `other` The value to add. `modulus` The modulus (must be ≥ 2). ## Return Value `(self + other) mod modulus`. ## Discussion Uses `addingReportingOverflow` to detect the 257-bit intermediate, then reduces modulo `modulus` via `dividingFullWidth` when overflow occurs or via a simple compare-and-subtract when it doesn’t. > Precondition: `modulus >= 2`.