code: def keep_indels(mut_df,
                  indel_len_col=True,
                  indel_type_col=True):
      
      # keep only frameshifts
      mut_df = mut_df[is_indel_annotation(mut_df)]
  
      if indel_len_col:
          # calculate length
          mut_df.loc[:, 'indel len'] = compute_indel_length(mut_df)
  
      if indel_type_col:
          is_ins = mut_df['Reference_Allele']=='-'
          is_del = mut_df['Tumor_Allele']=='-'
          mut_df['indel type'] = ''
          mut_df.loc[is_ins, 'indel type'] = 'INS'
          mut_df.loc[is_del, 'indel type'] = 'DEL'
  
      return mut_df
docs: Filters out all mutations that are not 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 indel
  
      Returns
      -------
      mut_df : pd.DataFrame
          mutations with only frameshift mutations kept