docs: def aa3_to_aa1(seq):
      """convert string of 3-letter amino acids to 1-letter amino acids
  
      >>> aa3_to_aa1("CysAlaThrSerAlaArgGluLeuAlaMetGlu")
      'CATSARELAME'
  
      >>> aa3_to_aa1(None)
  
      """
      if seq is None:
          return None
      return "".join(aa3_to_aa1_lut[aa3]
                     for aa3 in [seq[i:i + 3] for i in range(0, len(seq), 3)])