So one way to solve this would to be to add a `ensure-unique-seq` that wraps another generator, holds onto the head, and prevents duplicates being emitted into the seq. That might get costly (i.e. never finish) if the user asks for a large map with a key generator with not many distinct values, but I think we can handle that by firing an exception at them if that wrapper has `recurred` too many times (say like 3x the max number of elements they've asked for), and warning them that their use of it doesn't provide sufficient randomness.