code: function(map, key, value) {
  
      var list = [];
  
      if (map.has(key)) {
  
        list = map.get(key);
  
        if (!list) {
          list = [];
        }
  
  
      }
      list.push(value);
      map.set(key, list);
  
  }
docs: Generic method to add a new element in list which is indexed in a map
  So, value of each key is a list
  
  @param the map
  @param the key
  @param the new value to add to the list (the value) for that key