text: # This code is part of Qiskit.
  #
  # (C) Copyright IBM 2018, 2020.
  #
  # This code is licensed under the Apache License, Version 2.0. You may
  # obtain a copy of this license in the LICENSE.txt file in the root directory
  # of this source tree or at http://www.apache.org/licenses/LICENSE-2.0.
  #
  # Any modifications or derivative works of this code must retain this
  # copyright notice, and modified files need to carry a notice indicating
  # that they have been altered from the originals.
  
  """ estimator """
  
  from abc import ABC, abstractmethod
  
  
  class Estimator(ABC):
      """ Estimator class """
      @abstractmethod
      def fit(self, x, y):
          """ fit """
          raise NotImplementedError("Should have implemented this")
  
      @abstractmethod
      def decision_function(self, x):
          """ decision function """
          raise NotImplementedError("Should have implemented this")
  