First expression doesn't get cleared properly if you start a brand new one

```
500 [ENTER] # Should = 500, and does.
500 + 400 [ENTER] # Should = 900, and does NOT.

= 500 500 + 400 = 250400
```

Insect handles this just fine. I have no idea what's going wrong here. `bash` is such a pain.

```bash
$ insect
>>> 500

500

= 500

>>> 500 + 400

500 + 400

= 900
```

Current workaround is clearing when things stop making sense, but that's such a cop-out. The bug should get fixed.