import collections


def combine_dicts(dict_list):
  """Combines the dictionaries in dict_list.

  Args:
    dict_list: A list of dicts. Each dict maps integers to lists.

  Returns:
    combined: A dict that has for every key the 'combined' values of all dicts
    in dict list that have that key. Combining the values for a key amounts to
    concatenating the corresponding lists.
  """
  combined = collections.defaultdict(list)
  for d in dict_list:
    for k, v in d.items():
      combined[k].extend(v)
  return combined