Support where.not(condition)

This should be do-able and chain-able, i.e.

```
where(condition1).where(condition2).where.not(condition3)
```

In which case the third where is not changing any previous conditions.

Start at the where(condition) entry point and trace how chained conditions are represented. Verify that where(condition1).where(condition2).where.not(condition3) leaves the first two conditions unchanged, and add coverage demonstrating that chainable behavior.

Written by the indexing model from the issue text.