> Could the isaac generators also be Seedable<R: Rng> or something like that?

I thought about this, but it would preclude seeding with anything other than an `Rng`, so seeding with fixed numbers would be something like:

```rust
SeedableRng::from_seed(ReaderRng::new(MemReader::new(~[1,2,3])))
// rather than
SeedableRng::from_seed(&[1,2,3])
```

> Another option I suppose would be that there's a function for each implementation of StdRng which just "creates a new seed" of whatever the appropriate type is (or maybe calls a callback with a seed to prevent allocations

That seems reasonable, I'll experiment (later).