code: def add_fast(self, filepath, hashfn=None, force=False):
          
          if hashfn is None:
              hashfn = fast_hashes
          self.add(filepath, hashfn, force, shortcircuit=True)
docs: Bespoke function to add filepaths but set shortcircuit to True, which
          means only the first calculable hash will be stored. In this way only
          one "fast" hashing function need be called for each filepath.