var._decimalArith
private
var _decimalArith(string op)(const var rhs) const @safeDecimal arithmetic path for `+ - * /` with at least one FIXED_DECIMAL operand (design record D7): both operands become the wide runtime form (Int128 storage; integral operands convert exactly — signed via _tryToLong, unsigned through the exact ulong payload so values above long.max are accepted), the standalone checked operators run, and the result boxes FIXED_DECIMAL. Errors agree with the scalar operators (FixedDecimalException propagates); modulo and float, text or other kind mixes return var.init (the var convention for unsupported operand kinds).