Related to [discussion on discord](https://discord.com/channels/691052431525675048/742569353878437978/824731187724681289)
With const generics, it is now possible to write generic iterator over multiple entities at once.
This enables patterns of query iterations like
```rust
for [e1, e2, e3] in query.iter_combinations() {
// do something with relation of all three entities
}
```
The compiler is able to infer the correct iterator for given size of array, so either of those work
```rust
for [e1, e2] in query.iter_combinations() { ... }
for [e1, e2, e3] in query.iter_combinations() { ... }
```
This feature can be very useful for systems like collision detection.
When you ask for permutations of size K of N entities:
- if K == N, you get one result of all entities
- if K < N, you get all possible subsets of N with size K, without repetition
- if K > N, the result set is empty (no permutation of size K exist)
Co-authored-by: Carter Anderson <mcanders1@gmail.com>
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