def add_response(self, response, value):
        """
        Add response to staircase.

        Parameters
        ----------
        response : int or bool
            0 or 1.
        value : int or float
            Signal corresponding to response.
        """
        if value != "stop":
            self.X = pd.concat([self.X, pd.DataFrame({"Signal":[value]})])
            self.y = np.array(list(self.y) + [response])
            if len(set(list(self.y))) > 1:
                self.model = self.fit_model(self.X , self.y)
Add response to staircase.

        Parameters
        ----------
        response : int or bool
            0 or 1.
        value : int or float
            Signal corresponding to response.