A potential footgun

[tlienart]

```!
a = 5
```

```!
b = 5
a + b
```

```!
a = 10
```

If you then just do a change to b, then only that will get re-evaluated as well as cells below it (so that it's a=10 that will be used).

Solving this generally seems hard (Pluto style...) probably better to just

1. recommend users don't redefine variables carelessly within a page
2. if they really want to, that they put a !! on the first cell so that the whole page is evaluated on every change (because it would always get re-evaluated and, therefore, so will all the cells below it).