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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 def get_aa_letter(aa_code):
 """ 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'. """

    aa_letter = 'X'
    for key, val in standard_amino_acids.items():
        if val == aa_code:
            aa_letter = key

    return aa_letter