# Objective
Continuation of #17589 and #16547.
Slices have several methods that return iterators which themselves yield
slices, which we have not yet implemented.
An example use is `par_iter_many` style logic.
## Solution
Their implementation is rather straightforward, we simply delegate all
impls to `[T]`.
The resulting iterator types need their own wrappers in the form of
`UniqueEntitySliceIter` and `UniqueEntitySliceIterMut`.
We also add three free functions that cast slices of entity slices to
slices of `UniqueEntitySlice`.
These three should be sufficient, though infinite nesting is achievable
with a trait (like `TrustedEntityBorrow` works over infinite reference
nesting), should the need ever arise.
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