code: def extract_common(self, keys):
  		
  		keys = set(keys)
  		new = self.__class__()
  		intersection = self.intersection(keys)
  		for key in keys:
  			dict.__setitem__(new, key, _shallowcopy(intersection))
  			dict.__setitem__(new.offsets, key, self.offsets[key])
  		return new
docs: Return a new segmentlistdict containing only those
  		segmentlists associated with the keys in keys, with each
  		set to their mutual intersection.  The offsets are
  		preserved.