Can add other list items to a LIST variable, polluting it

The following does not seem like a good idea...

```
LIST Trees = Pine, Ash, Elm, Beech
LIST Flowers = Roses, Daisies

{LIST_ALL(Trees)}

~ Trees = Roses

{LIST_ALL(Trees)}
```

...as the addition of the "Roses" element causes the meaning of LIST_ALL to change in quite an unexpected way. (I have been doing this without realising in HV with some serious results!!)