def is_tf(df):
    """
    Determines if a string is a 'boolean string': 't', 'f', 'True', or 'False'.
    Args:
        df (pandas.DataFrame): A dataframe that contains strings.
    Returns:
        pandas.DataFrame: A dataframe of booleans that is True if the entry
            is any of the valid 'boolean string' values.
    """
    return (df == 't') | (df == 'f') | (df == 'True') | (df == 'False')