def BinPow(a, n, fun):
    if n == 1:
        return fun(a)
    else:
        def BinPow1(a1, n1, fun1=fun):
            if n == 1:
                return fun1(a)
            else:
                return BinPow1(fun1(a1), n1 - 1)
        return(BinPow1(fun1(a1), n - 1))

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