code: def test_split_on_header(
      markdown="""A paragraph
  
  # H1 Header
  
  ## H2 Header
  
  Another paragraph
  """,
  ):
      fmt = {"extension": ".md", "split_at_heading": True}
      nb = jupytext.reads(markdown, fmt)
      assert nb.cells[0].source == "A paragraph"
      assert nb.cells[1].source == "# H1 Header"
      assert nb.cells[2].source == "## H2 Header\n\nAnother paragraph"
      assert len(nb.cells) == 3
      markdown2 = jupytext.writes(nb, fmt)
      compare(markdown2, markdown)
  
imports: from nbformat.v4.nbbase import new_code_cell, new_markdown_cell, new_notebook, new_raw_cell
  import jupytext
  from jupytext.combine import combine_inputs_with_outputs
  from jupytext.compare import compare, compare_cells, compare_notebooks
label_desc: Use image node_id 6 for calling a global function with example usage: test_split_on_header(markdown) without return types