new_contents: import random
  
  from feature import *
  
  
  def test_transform():
      """Test if custom features work."""
  
      def transform(array):
          """Turns the (n,2) array into a (n,4) array."""
          new = Array(columns="abcd")
          for _ in array:
              new.append([1, 2, 3, 4])
          return new
  
      group = Group({"a": Numerical(), "b": Numerical()}, transform=transform)
  
      for _ in range(10):
          group.set_a(random.random())
          group.set_b(random.random())
          group.push()
  
      array = group.array()
      assert array.shape == (10, 4)
  
subject: add transform test