def wos(wos):
    """Transform the wos dataset according to what was found in the explaratory
    analyses, available on sp_np.ipynb

    1. Remove duplicates based on columns: 'Source', 'Year Published', 'Volume',
    'Issue', 'Page Span' columns;
    2. Select only 'Source' and 'Year Published' columns
    3. Rename these two columns to 'Journal' and 'Year'
    4. Add a column named 'Source', with all rows = 'WoS/Zoological Records'

    Args:
        wos (pandas.DataFrame): Unchanged dataframe from the Web of Science/Zoo-
        logical records parser

    Returns:
        pandas.DataFrame: Transformed wos dataframe
    """
    # Removing duplicates
    wos.drop_duplicates(subset=['Source', 'Year Published', 'Volume',
        'Issue', 'Page Span'], keep=False, inplace=True)

    # Selecting only the columns 'Source' and 'Year Published'
    wos = wos[['Source', 'Year Published']]

    # Renaming columns
    wos.rename(columns={'Source': 'Journal', 'Year Published': 'Year'},
        inplace=True)

    # Adding column 'Source', filling all rows = 'WoS/Zoological Records'
    wos['Source'] = 'WoS/Zoological Records'

    return wos