code: def _to_tonnetz(chromagram):
      
      if np.sum(np.abs(chromagram)) == 0.:
          # The input is an empty chord, return zero. 
          return np.zeros(6)
      
      _tonnetz = np.dot(__TONNETZ_MATRIX, chromagram)
      one_norm = np.sum(np.abs(_tonnetz))  # Non-zero value
      _tonnetz = _tonnetz / float(one_norm) # Normalize tonnetz vector
      return _tonnetz
docs: Project a chromagram on the tonnetz.
  
      Returned value is normalized to prevent numerical instabilities.