code: def concat(xss):
      r"""
      Concat list of list to be a list.
      Usage::
  
          concat([[1, 2], [2, 3]]) == [1, 2, 2, 3]
  
      :param list xss: the list to be concat
      """
      from functools import reduce
  
      if len(xss) == 0:
          return []
      return reduce(lambda x, y: x + y, xss)
  
label_desc: Use image node_id 1 for calling a global function with example usage: concat(xss) and returns: reduce, list