code: def _merge_lists(nested_list, high_level_indices=None):
      """
      merge elements of lists (of a nested_list) into one single tuple with elements
      sorted in ascending order.
  
      Parameters
      ----------
      nested_list: List
          a  list whose elements must be list as well.
  
      high_level_indices: list or tuple, default None
          a list or tuple that contains integers that are between 0 (inclusive) and
          the length of `nested_lst` (exclusive). If None, the merge of all
          lists nested in `nested_list` will be returned.
  
      Returns
      -------
      out: tuple
          a tuple, with elements sorted in ascending order, that is the merge of inner
          lists whose indices are provided in `high_level_indices`
  
      Example:
      nested_list = [[1],[2, 3],[4]]
      high_level_indices = [1, 2]
      >>> _merge_lists(nested_list, high_level_indices)
      (2, 3, 4) # merging [2, 3] and [4]
      """
      if high_level_indices is None:
          high_level_indices = list(range(len(nested_list)))
  
      out = []
      for idx in high_level_indices:
          out.extend(nested_list[idx])
  
      return tuple(sorted(out))
  
imports: from copy import deepcopy
  import numpy
  from sklearn.model_selection import cross_val_score
label_desc: Use image node_id 1 for calling a global function with example usage: _merge_lists(nested_list, high_level_indices) and returns: tuple