@mbj I'd like to get your input on these mutations. I wonder if they may be unkillable.

What I was thinking, instead of replacing the nodes with true/false/nil, I could just flip the boolean context, eg:

```ruby
let(:source) { 'true and false' }

let(:mutations) do
  mutations = []
  mutations << 'nil'
  mutations << 'true'
  mutations << 'false'

  mutations << 'true or false'

  mutations << '!true and false'
  mutations << 'true and !false'
end
```

This seems like a safer mutation and should be killable. wdyt?