overwritingMethodWrapper = function () {
    // Setting the `ssfi` flag to `overwritingMethodWrapper` causes this
    // function to be the starting point for removing implementation frames from
    // the stack trace of a failed assertion.
    //
    // However, we only want to use this function as the starting point if the
    // `lockSsfi` flag isn't set.
    //
    // If the `lockSsfi` flag is set, then either this assertion has been
    // overwritten by another assertion, or this assertion is being invoked from
    // inside of another assertion. In the first case, the `ssfi` flag has
    // already been set by the overwriting assertion. In the second case, the
    // `ssfi` flag has already been set by the outer assertion.
    if (!flag(this, 'lockSsfi')) {
      flag(this, 'ssfi', overwritingMethodWrapper);
    }

    // Setting the `lockSsfi` flag to `true` prevents the overwritten assertion
    // from changing the `ssfi` flag. By this point, the `ssfi` flag is already
    // set to the correct starting point for this assertion.
    var origLockSsfi = flag(this, 'lockSsfi');
    flag(this, 'lockSsfi', true);
    var result = method(_super).apply(this, arguments);
    flag(this, 'lockSsfi', origLockSsfi);

    if (result !== undefined) {
      return result;
    }

    var newAssertion = new chai.Assertion();
    transferFlags(this, newAssertion);
    return newAssertion;
  }
### .overwriteMethod(ctx, name, fn)

Overwrites an already existing method and provides
access to previous function. Must return function
to be used for name.

    utils.overwriteMethod(chai.Assertion.prototype, 'equal', function (_super) {
      return function (str) {
        var obj = utils.flag(this, 'object');
        if (obj instanceof Foo) {
          new chai.Assertion(obj.value).to.equal(str);
        } else {
          _super.apply(this, arguments);
        }
      }
    });

Can also be accessed directly from `chai.Assertion`.

    chai.Assertion.overwriteMethod('foo', fn);

Then can be used as any other assertion.

    expect(myFoo).to.equal('bar');

@param {Object} ctx object whose method is to be overwritten
@param {String} name of method to overwrite
@param {Function} method function that returns a function to be used for name
@namespace Utils
@name overwriteMethod
@api public