Actions

From the user $U$ it is required that he is able to press the keys in a pre-defined order. If this happens then the door will change from being closed to being open. An action has a dynamic character. An action will be recognized by its effect on the environment: giving a certain set of properties $\pi = \{ \pi_{1}, ..., \pi_{k} \}$ during a time interval (t,t') an action $\alpha$ changes at least one property $\pi_{i}$ such that we have another set $\pi' = \{\pi_{1}, ..., \pi_{k} \}$ where at least $\pi_{i}$ has changed.

$$action : 2^{\Pi} \longmapsto 2^{\Pi}$$ (4.4)

In this sense an action $\alpha$ is a kind of an atomic transition, a perceivable change of static properties characterizing a scene, a situation, a scenario.

But there can also be actions of other actors than the user $U$. If e.g. a user presses a key 'A' then will the key 'A' usually after releasing the key 'A' 'automatically' become again 'not pressed'. One could also say that the system 'acts' by 're-pressing' the key 'A' 'back'.