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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_code(aa_letter):
 """ Get three-letter aa code if possible. If not, return None. If three-letter code is None, will have to find this later from the filesystem. Parameters aa_letter : str One-letter amino acid code. Returns ------- aa_code : str, or None Three-letter aa code. """

    aa_code = None
    if aa_letter != 'X':
        for key, val in standard_amino_acids.items():
            if key == aa_letter:
                aa_code = val
    return aa_code