I'm trying to generate a `Seq` of random options where consecutive repeats are not allowed:

```perl6
> (<F R U>.roll(*).grep(* ne *))[^10]
((R F) (F U) (R F) (F R) (U R) (R F) (R F) (U F) (R U) (U R))

```

Why does using `grep` in this way result in nested pairs? And how can I achieve what I'm looking for? Using `.flat` above will still allow consecutive repeats.

```perl6
( R U F U F U R F R U ... )

```