code: def keep_frameshifts(mut_df,
                       indel_len_col=True):
      
      # 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
docs: 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