I think the body of the spec you have is fine (although, "callable responder" is a little awkwardly worded -- I might go with something like `it "supports block implementations"`).  My point was that it would be good to have another spec like:

``` ruby
it "does not set an expectation that the message will be received" do
  ::RSpec::Mocks.allow_message(subject, :message)
  expect { verify subject }.not_to raise_error
end
```