Stories (aka user stories or scenarios) are told in terms of input messages (or triggers) and expected messages or values...
On one beautiful summer eve, a guest arrived, so let's send that message:
Naturally, the lights must have come on and Jane was greeted:
Let's see if John is greeted - he shouldn't be.
The implementation of these rules is in a-lights-spec, check it out!
Play with this: change some values, for instance
Jim and see how things look when there are errors.