sample_spec.rb

      before(:each) do
        # doesn't work for what I need.
        # allow_any_instance_of(Edge).to receive(:freeze).and_return(nil)

        @c = Converter.new name: 'c'
        @edge_in = instance_double(Edge, from: double(), to: @c, freeze: nil)
        @edge_out = instance_double(Edge, from: @c, to: double(), freeze: nil)
        @c.attach_edge!(@edge_in).attach_edge!(@edge_out)

      end

      it 'pings incoming nodes' do

        expect(@edge_in).to receive(:test_ping?)
        @edge_out.as_null_object
        @c.trigger!

      end