Also, I'm starting to get confused about the initial purpose of the ```description_when_negated ```:

these all pass, without the ```description_when_negated ``` implementation:

``` ruby
describe 'failure messages for built in matchers', :focus do
      it "be" do
        matcher = ~(be_odd)
        matcher.matches?(3)
        expect(matcher.failure_message).to eq 'expected odd? to return false, got true'
      end

      it "be_kind_of" do
        matcher = ~(be_kind_of(Integer))
        matcher.matches?(3)
        expect(matcher.failure_message).to eq 'expected 3 not to be a kind of Integer'
      end

      it "be_between" do
        matcher = ~be_between(1, 5)
        matcher.matches?(3)
        expect(matcher.failure_message).to eq 'expected 3 not to be between 1 and 5 (inclusive)'
      end

      it "cover" do
        matcher = ~cover(2)
        matcher.matches?(1..3)
        expect(matcher.failure_message).to eq 'expected 1..3 not to cover 2'
      end

      it "include" do
        matcher = ~include(2)
        matcher.matches?([1, 2, 3])
        expect(matcher.failure_message).to eq 'expected [1, 2, 3] not to include 2'
      end
    end
```

@myronmarston can you please reiterate you're thoughts regarding ```description_when_negated ```. 
I need some guidance :-)