public void ExactlyWhatYouAreNotSupposedToDo()
        {
            // i r ImmutableArray<T>.Builder nao, look at me
            ulong[] origBuf = { 1, 2, 3, 4 };
            ImmutableArray <ulong> fakeImmutableBuf = origBuf.AsImmutableArrayDangerous();

            // we should get the same values.
            Assert.Equal(origBuf, fakeImmutableBuf);

            // it should also be exactly the same instance... this MUST be copy-free, or else it's
            // not worth it.
            Assert.Same(origBuf, fakeImmutableBuf.AsRegularArrayDangerous());

            // this is exactly what you're not supposed to do.
            ++origBuf[2];
            Assert.Equal(origBuf, fakeImmutableBuf);
        }