# Objective
Fixes#7286. Both `App::add_sub_app` and `App::insert_sub_app` are rather redundant. Before 0.10 is shipped, one of them should be removed.
## Solution
Remove `App::add_sub_app` to prefer `App::insert_sub_app`.
Also hid away `SubApp::extract` since that can be a footgun if someone mutates it for whatever reason. Willing to revert this change if there are objections.
Perhaps we should make `SubApp: Deref<Target=App>`? Might change if we decide to move `!Send` resources into it.
---
## Changelog
Added: `SubApp::new`
Removed: `App::add_sub_app`
## Migration Guide
`App::add_sub_app` has been removed in favor of `App::insert_sub_app`. Use `SubApp::new` and insert it via `App::add_sub_app`
Old:
```rust
let mut sub_app = App::new()
// Build subapp here
app.add_sub_app(MySubAppLabel, sub_app);
```
New:
```rust
let mut sub_app = App::new()
// Build subapp here
app.insert_sub_app(MySubAppLabel, SubApp::new(sub_app, extract_fn));
```
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