code: func NewTarSum(r io.Reader, dc bool, v Version) (TarSum, error) {
  	return NewTarSumHash(r, dc, v, DefaultTHash)
  }
docs: // NewTarSum creates a new interface for calculating a fixed time checksum of a
  // tar archive.
  //
  // This is used for calculating checksums of layers of an image, in some cases
  // including the byte payload of the image's json metadata as well, and for
  // calculating the checksums for buildcache.