code: def underscore_join(iterable):
      """Create flattened column names from multiindex tuple.
  
      Parameters
      ----------
      iterable : an iterable
  
      Returns
      -------
      str, for an iterable (x1, x2, ..., xn), returns the string
          str(x1) + "__" + str(x2) + "__" + str(x3) + ... + "__" + str(xn)
      """
      iterable_as_str = [str(x) for x in iterable]
      return "__".join(iterable_as_str)
  
label_desc: Use image node_id 1 for calling a global function with example usage: underscore_join(iterable) and returns: str