code: function assertCalled (n) {
      new Assertion(this._obj).to.be.spy;
      var spy = this._obj.__spy;
  
      if (n) {
        this.assert(
            spy.calls.length === n
          , 'expected ' + this._obj + ' to have been called #{exp} but got #{act}'
          , 'expected ' + this._obj + ' to have not been called #{exp}'
          , n
          , spy.calls.length
        );
      } else {
        this.assert(
            spy.called === true
          , 'expected ' + this._obj + ' to have been called'
          , 'expected ' + this._obj + ' to not have been called'
        );
      }
    }
docs: # .called
  
  Assert that a spy has been called. Does not negate to allow for
  pass through language.
  
  @api public