code: def properties_for(self, index):
          
          return vectorize(lambda i: [prop for prop in self.properties() if i in self[prop]], otypes=[list])(index)
docs: Returns a list of properties, such that each entry in the list corresponds
          to the element of the index given.
  
          Example:
          let properties: 'one':[1,2,3,4], 'two':[3,5,6]
  
          >>> properties_for([2,3,5])
          [['one'], ['one', 'two'], ['two']]