code: def _nbytes(self, deep=False):
          
  
          # for implementations with no useful getsizeof (PyPy)
          objsize = 24
  
          level_nbytes = sum(i.memory_usage(deep=deep) for i in self.levels)
          label_nbytes = sum(i.nbytes for i in self.codes)
          names_nbytes = sum(getsizeof(i, objsize) for i in self.names)
          result = level_nbytes + label_nbytes + names_nbytes
  
          # include our engine hashtable
          result += self._engine.sizeof(deep=deep)
          return result
docs: return the number of bytes in the underlying data
          deeply introspect the level data if deep=True
  
          include the engine hashtable
  
          *this is in internal routine*