# Returns `self` raised to the power of `e`.
    #
    #     assert(3.to_bi ** 2 == 9.to_bi)
    fun **(e: Int): BigInt do
        var res = new NativeMPZ
        var pow = new UInt64
        pow.set_si e
        val.pow_ui(res, pow)
        pow.free
        return new BigInt(res)
    end