Comment by aapoalas

Guidance towards correct usage: eg. If you allow `a[10] = 2` and just make the Array dense, the user might not even realise the difference and will assume it's sparse. Next they perform `a[2636173747] = 3` and clog up the entire VM memory or just plain crash it from OOM. Since it's likely that the small indexes appear in testing and the large indexes appear in production, it is better to make the misunderstanding an explicit error and move it "leftwards" in time, so that it doesn't crash production at an inopportune moment.