Sock: when the spec generates the mock

HERE BE DRAGONS.

This is an initial experiment. Think about it. Talk about it. But if you use it
in anger, expect it to make you angry.

HERE BE DRAGONS.

Consider the following test, for a calculator class:

it "returns 6 for inputs 3 and 2" do
calculator = Calculator.new
calculator.push(2)
calculator.push(3)
calculator.mul.should == 6
end

And this one, for something that uses the calculator class:

it "uses the calculator to multiply 3 and 2" do
calculator.should_receive(:push).with(2)
calculator.should_receive(:push).with(3)
calculator.should_receive(:mul).and_return(6)
CalculatorDriver.run(calculator)
end

But in the real world, eventually the calculator class will change and all its
consumers will break in real code, but the consumer's tests will keep right on
passing. Bad. If only the mock for the latter test could be generated by the
former...

Oh look. It can.