code: func (r *Reader) ReadStateFile() (*statefile.File, error) {
  	for _, file := range r.zip.File {
  		if file.Name == tfstateFilename {
  			r, err := file.Open()
  			if err != nil {
  				return nil, fmt.Errorf("failed to extract state from plan file: %s", err)
  			}
  			return statefile.Read(r)
  		}
  	}
  	return nil, statefile.ErrNoState
  }
docs: // ReadStateFile reads the state file embedded in the plan file.
  //
  // If the plan file contains no embedded state file, the returned error is
  // statefile.ErrNoState.