function Assertion (obj, msg, ssfi, lockSsfi) {
    flag(this, 'ssfi', ssfi || Assertion);
    flag(this, 'lockSsfi', lockSsfi);
    flag(this, 'object', obj);
    flag(this, 'message', msg);

    return util.proxify(this);
  }
# .use(function)

Provides a way to extend the internals of Chai.

@param {Function}
@returns {this} for chaining
@api public