def add_preexisting(df):
    """
    desc : denotes whether or not a user has a pre-existing heart condition (high blood pressure or heart disease)
    args:
        df (pd.DataFrame) : stroke dataframe
    returns:
        df (pd.DataFrame) : stroke dataframe with pre_existing column
    """
    stroke_data = df.copy()
    stroke_data["pre_existing"] = (stroke_data['hypertension'] + stroke_data['heart_disease']).astype("bool")
    return stroke_data