function add(x, y) {
  x + y;
}
function subtract(x, y) {
  x - y;
}
function divide(x, y) {
  x / y;
}
function multiply(x, y) {
  x * y;
}

module.exports.add = add;
module.exports.subtract = subtract;
module.exports.divide = divide;
module.exports.multiply = multiply;