def BinPow(a, n, fun):
    if n == 1:
        return fun(a)
    else:
        def BinPow1(a1, n1):
            if n1 == 1:
                return fun(a1)
            else:
                return BinPow1(fun(a1), n1 - 1)

        return (BinPow1(fun(a), n - 1))


print(BinPow("Se", 7, str.__add__))