def _simplify_features(df):
    """ Remove 'standard error' columns from
    the breast cancer dataset for simplicity
    and replace spaces with underscores.
    """
    simple = [col for col in df.columns if 'error' not in col]
    simple_df = df[simple]
    simple_df.columns = [col.replace(' ', '_') for col in simple]
    
    return simple_df