code: def group_list(items):
      
      groups = defaultdict(list)
      for key, value in items:
          groups[key].append(value)
      return groups
docs: Items in the list are grouped into a dictionary
  
      Items should be a list of (key, value) pairs
  
      >>> grouped = group_list([(1,0), (2,0), (1,1)])
      >>> grouped[1] == [0,1]
      True