code: def filter_transition_coefs(transition_coef, indices):
      
      indices = np.array(indices)
      rows = transition_coef[indices]
      return rows[:,indices]
docs: >>> coef = np.array([[0, 1, 2], [3, 4, 5], [6, 7, 8]])
      >>> filter_transition_coefs(coef, [0])
      array([[0]])
      >>> filter_transition_coefs(coef, [1, 2])
      array([[4, 5],
             [7, 8]])
      >>> filter_transition_coefs(coef, [2, 0])
      array([[8, 6],
             [2, 0]])
      >>> filter_transition_coefs(coef, [0, 1, 2])
      array([[0, 1, 2],
             [3, 4, 5],
             [6, 7, 8]])