docs: def create_unique_id(prefix, existing_ids):
      """Return a unique string ID from the prefix.
  
      First check if the prefix is itself a unique ID in the set-like parameter
      existing_ids. If not, try integers in ascending order appended to the
      prefix until a unique ID is found.
      """
      if prefix in existing_ids:
          suffix = 1
          while True:
              new_id = '{}_{}'.format(prefix, suffix)
              if new_id not in existing_ids:
                  return new_id
              suffix += 1
  
      return prefix