code: def downgrade(nb):
      """Convert a v2 notebook to v1.
  
      Parameters
      ----------
      nb : NotebookNode
          The Python representation of the notebook to convert.
      """
      msg = "Downgrade from notebook v2 to v1 is not supported."
      raise Exception(msg)
  
imports: from __future__ import annotations
  from .nbbase import new_code_cell, new_notebook, new_text_cell, new_worksheet
label_desc: Use image node_id 2 for calling a global function with example usage: downgrade(nb) without return types