如何重构Mocha BDD测试？

// calculator.js
function add(a, b) {
  return a + b;
}

function subtract(a, b) {
  return a - b;
}

// test.js
const assert = require('assert');
const { add, subtract } = require('./calculator');

describe('Calculator', function() {
  it('should add two numbers', function() {
    assert.strictEqual(add(1, 2), 3);
  });

  it('should subtract two numbers', function() {
    assert.strictEqual(subtract(3, 1), 2);
  });
});

// test.js
const assert = require('assert');
const { add, subtract } = require('./calculator');

describe('Calculator', function() {
  beforeEach(function() {
    // 公共的初始化逻辑（如果有）
  });

  afterEach(function() {
    // 公共的清理逻辑（如果有）
  });

  describe('#add', function() {
    it('should return the sum of two numbers', function() {
      assert.strictEqual(add(1, 2), 3);
    });

    it('should handle negative numbers', function() {
      assert.strictEqual(add(-1, -2), -3);
    });
  });

  describe('#subtract', function() {
    it('should return the difference of two numbers', function() {
      assert.strictEqual(subtract(3, 1), 2);
    });

    it('should handle negative results', function() {
      assert.strictEqual(subtract(1, 3), -2);
    });
  });
});