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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 keep_frameshifts(mut_df, indel_len_col=True):
 """Filters out all mutations that are not frameshift indels. Requires that one of the alleles have '-' indicating either an insertion or deletion depending if found in reference allele or somatic allele columns, respectively. Parameters mut_df : pd.DataFrame mutation input file as a dataframe in standard format indel_len_col : bool whether or not to add a column indicating the length of the frameshift Returns ------- mut_df : pd.DataFrame mutations with only frameshift mutations kept """

    # keep only frameshifts
    mut_df = mut_df[is_frameshift_annotation(mut_df)]

    if indel_len_col:
        # calculate length
        mut_df.loc[:, 'indel len'] = compute_indel_length(mut_df)
    return mut_df