def write_r(df, f, sep=",", index_join="@", columns_join="."):
    """
    Export dataframe in a format easily importable to R

    Index fields are joined with "@" and column fields by "." by default.
    :param df:
    :param f:
    :param index_join:
    :param columns_join:
    :return:
    """

    df = df.copy()
    df.index = ["@".join([str(s) for s in v]) for v in df.index.values]
    df.columns = [".".join([str(s) for s in v]) for v in df.index.values]
    df.to_csv(f, sep=sep)