@myronmarston in order to make ```description_when_negated``` I had to edit ```BaseMatcher```:
``` ruby
      def failure_message_when_negated
          assert_ivars :@actual
          if respond_to?(:description_when_negated)
            "expected #{@actual.inspect} to #{description_when_negated}" # this is the reason for the change
          else
            "expected #{@actual.inspect} to not #{description}" # default negation
          end
        end
```
see: https://github.com/rspec/rspec-expectations/pull/505/files#r11254470

this allows ```description_when_negated``` to be more flexible, since I can now do:
```
expected 2.5 to be a value not between 2 and 3
```
which is what you requested in: https://github.com/rspec/rspec-expectations/pull/505#discussion_r11121527

in other cases, I changed it to be consistent with this form.

it is my understanding that both forms are grammatically correct. (although, "not to" may be more prevalent)