code: def get_aa_letter(aa_code):
      
      aa_letter = 'X'
      for key, val in standard_amino_acids.items():
          if val == aa_code:
              aa_letter = key
  
      return aa_letter
docs: Get one-letter version of aa_code if possible. If not, return 'X'.
  
      Parameters
      ----------
      aa_code : str
          Three-letter amino acid code.
  
      Returns
      -------
      aa_letter : str
          One-letter aa code.
          Default value is 'X'.