Please bear in mind https://github.com/cucumber/cucumber-ruby-core/issues/20. I'd like to add as few couplings to Gherkin as possible from cucumber-core as it's likely to be changed significantly in the future. The `Gherkin::TagExpression` class is useful for evaluating complex boolean expressions, but do we do that in tag filters? Seems like a `not` operator is about as complex as it gets. Might not be worth adding the dependency.