def clean(df):
    """Cleansing: a data pre-processing step. Usually, getting rid of garbage
    such as undesired characters.

    Cleansing must be a set of operations independent of data splitting.

    Parameters
    ----------
    df: pandas dataframe

    Returns
    -------
    pandas dataframe
        Cleansed dataframe

    Examples
    --------

    Raises
    ------

    Notes
    -----

    """

    return df


def normalize(df):
    """Normalization: a data pre-processing step. Usually, making adjusting
    loser and upper casing, abbrevations, word order, and so on.

    Normalization must be a set of operations independent of data splitting.

    Parameters
    ----------
    df: pandas dataframe

    Returns
    -------
    pandas dataframe
        Normalized dataframe

    Examples
    --------

    Raises
    ------

    Notes
    -----

    """

    return df
