code: def test_cum_fold() -> None:
      df = pl.DataFrame(
          {
              "a": [1, 2, 3, 4],
              "b": [5, 6, 7, 8],
              "c": [10, 20, 30, 40],
          }
      )
      result = df.select(
          pl.cum_fold(pl.lit(0), lambda a, b: a + b, pl.all())
      )
      expected = pl.DataFrame(
          {
              "cum_fold": [
                  {"a": 1, "b": 6, "c": 16},
                  {"a": 2, "b": 8, "c": 28},
                  {"a": 3, "b": 10, "c": 40},
                  {"a": 4, "b": 12, "c": 52},
              ]
          }
      )
      assert_frame_equal(result, expected)
  
imports: import polars
  from polars.testing import assert_frame_equal
label_desc: Use image node_id 2 for calling a global function with example usage: test_cum_fold() without return types