def make_aa_to_codon_backtable(codontable):
    """Convert a codontable for use with convert_seq_to_seqspace()

    Returns a codontable (dict) in the format `{'F': ['TTT', 'TTC'], 'L': ['TTA', ...`
    
    Parameters
    ----------

    codontable
      dict of codons in the format `{'TTT': 'F', 'TTC': 'F', 'TTA': 'L', ...`
    """

    aa_list = list(set(codontable.values()))
    backtable = {key: [] for key in aa_list}
    for codon, aa in codontable.items():
        backtable[aa] = backtable[aa] + [codon]

    return backtable