title: How do I spec instance methods on an abstract class in Crystal?
body: <p>Let's say I have an abstract class which has a non-abstract instance method for children to inherit:</p>
  
  <pre><code># - abstract.cr
  abstract class Abstract
    def foo
      2
    end
  end
  </code></pre>
  
  <p>How do I write a spec for this?</p>
  
  <pre><code># - abstract_spec.cr
  
  it "returns 2 from #foo" do
    Abstract.instance.foo.should eq 2 #???
  end
  </code></pre>