node.js: Stubbing with gently and assert modules

gistfile1.js

var gently = new (require('gently'))
  , assert = require('assert')

function Dog() {
}

Dog.prototype.seeCat = function() {
  console.log('seeing cat...');
  this.bark('whuf, whuf');
  this.run();
}

Dog.prototype.bark = function(bark) {
  console.log('real: ' + bark)
}

var dog = new Dog();

gently.expect(dog, 'bark', function(bark) {
  assert.equal(bark, 'whuf, whuf');
  console.log('fake: ' + bark)
});

gently.expect(dog, 'run');

dog.seeCat(); // executing dog action

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Both "expect" calls fake the real Dog methods. Even Dog.prototype.run() that does not exist.