new_contents: import pytest
  import os
  import numpy as np
  
  import sys
  myPath = os.path.dirname(os.path.abspath(__file__))
  sys.path.insert(0, os.path.join(myPath, '..'))
  
  from DBNlogic.sets import exists, DataSet, MNIST, SmallerMNIST
  
  def test_exists_true():
      """The function `exists` returns True if
      given the path of an existing file."""
      file = open('test.test', 'w+')
      file.close()
      assert exists('test.test')
      os.remove('test.test')
  
  def test_exists_false():
      """The function `exists` returns False if
      given the path of an unexisting file."""
      assert exists('test.test') == False
  
subject: Add tests for back end