code: def is_frameshift_len(mut_df):
      
      # calculate length, 0-based coordinates
      #indel_len = mut_df['End_Position'] - mut_df['Start_Position']
      if 'indel len' in mut_df.columns:
          indel_len = mut_df['indel len']
      else:
          indel_len = compute_indel_length(mut_df)
  
      # only non multiples of 3 are frameshifts
      is_fs = (indel_len%3)>0
  
      # make sure no single base substitutions are counted
      is_indel = (mut_df['Reference_Allele']=='-') | (mut_df['Tumor_Allele']=='-')
      is_fs[~is_indel] = False
      return is_fs
docs: Simply returns a series indicating whether each corresponding mutation
      is a frameshift.
  
      This is based on the length of the indel. Thus may be fooled by frameshifts
      at exon-intron boundaries or other odd cases.
  
      Parameters
      ----------
      mut_df : pd.DataFrame
          mutation input file as a dataframe in standard format
  
      Returns
      -------
      is_fs : pd.Series
          pandas series indicating if mutaitons are frameshifts