code: def multi_dict(pairs):
      
      result = collections.defaultdict(list)
      for k, v in pairs:
          result[k].append(v)
      return result
docs: Given a set of key value pairs, create a dictionary.
      If a key occurs multiple times, stack the values into an array.
  
      Can be called like the regular dict(pairs) constructor
  
      Parameters
      ----------
      pairs: (n,2) array of key, value pairs
  
      Returns
      ----------
      result: dict, with all values stored (rather than last with regular dict)