def is_frameshift_len(mut_df):
    """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
    """
    # 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
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