old_contents: /*!
   * arr-diff <https://github.com/jonschlinkert/arr-diff>
   *
   * Copyright (c) 2014 Jon Schlinkert, contributors.
   * Licensed under the MIT License
   */
  
  'use strict';
  
  var should = require('should');
  var diff = require('./');
  
  describe('diff', function () {
    it('should diff array:', function () {
      diff(['a', 'b', 'c'], ['b', 'c', 'e']).should.eql(['a']);
      diff(['foo', 'b', 'c', 'e', 'bar'], ['b', 'foo', 'e']).should.eql(['bar', 'c']);
      diff(['foo', 'foo'], ['a', 'b', 'c']).should.eql(['foo', 'foo']);
      diff(['foo'], ['a', 'b', 'c']).should.eql([]);
    });
  
    it('should return an empty array if no unique elements are in the first array:', function () {
      diff(['foo'], ['a', 'b', 'c', 'foo']).should.eql([]);
    });
  });
  
new_contents: /*!
   * arr-diff <https://github.com/jonschlinkert/arr-diff>
   *
   * Copyright (c) 2014 Jon Schlinkert, contributors.
   * Licensed under the MIT License
   */
  
  'use strict';
  
  var should = require('should');
  var diff = require('./');
  
  describe('diff', function () {
    it('should diff array:', function () {
      diff(['a', 'b', 'c'], ['b', 'c', 'e']).should.eql(['a']);
      diff(['x', 'b', 'c', 'e', 'y'], ['b', 'x', 'e']).should.eql(['c', 'y']);
      diff(['x', 'x'], ['a', 'b', 'c']).should.eql(['x', 'x']);
      diff(['x'], ['a', 'b', 'c']).should.eql(['x']);
    });
  
    it('should include duplicates:', function () {
      diff(['x', 'b', 'b', 'b', 'c', 'e', 'y'], ['x', 'e']).should.eql(['b', 'b', 'b', 'c', 'y']);
    });
  
    it('should diff elements from multiple arrays:', function () {
      diff(['a', 'b', 'c'], ['a'], ['b']).should.eql(['c']);
    });
  
    it('should return an empty array if no unique elements are in the first array:', function () {
      diff(['a'], ['a', 'b', 'c']).should.eql([]);
    });
  
    it('should return the first array if the second array is empty:', function () {
      diff(['a', 'b', 'c'], []).should.eql(['a', 'b', 'c']);
    });
  
    it('should return the first array if the second array is null or undefined:', function () {
      diff(['a', 'b', 'c'], null).should.eql(['a', 'b', 'c']);
      diff(['a', 'b', 'c']).should.eql(['a', 'b', 'c']);
    });
  });
  
subject: update test