# Objective
Add a way to efficiently replace a set of specifically related entities
with a new set.
Closes#18041
## Solution
Add new `replace_(related/children)` to `EntityWorldMut` and friends.
## Testing
Added a new test to `hierarchy.rs` that specifically check if
`replace_children` actually correctly replaces the children on a entity
while keeping the original one.
---
## Showcase
`EntityWorldMut` and `EntityCommands` can now be used to efficiently
replace the entities a entity is related to.
```rust
/// `parent` has 2 children. `entity_a` and `entity_b`.
assert_eq!([entity_a, entity_b], world.entity(parent).get::<Children>());
/// Replace `parent`s children with `entity_a` and `entity_c`
world.entity_mut(parent).replace_related(&[entity_a, entity_c]);
/// `parent` now has 2 children. `entity_a` and `entity_c`.
///
/// `replace_children` has saved time by not removing and reading
/// the relationship between `entity_a` and `parent`
assert_eq!([entity_a, entity_c], world.entity(parent).get::<Children>());
---------
Co-authored-by: Alice Cecile <alice.i.cecile@gmail.com>
7cd729cf1a