Huh. Good point! That means servo's only use of `mangle`:

```
                self.idmap.mangle(new_id, abstract_self,
                                  |_, new_node: AbstractNode| -> AbstractNode {
                                      new_node
                                  },
                                  |_, old_node: &mut AbstractNode, new_node: AbstractNode| {
                                      *old_node = new_node;
                                  });
```

should have been:

```
                self.idmap.insert(new_id, abstract_node);
```