Concurrent Actions

Some effects (e.g., clapping) can be produced only if certain actions (e.g., moving left hand and moving right hand) are done concurrently. On the other hand, performing some actions concurrently can prove disastrous (e.g. kicking with both legs or pushing and pulling the same object).

The preconditions for kicking with the left leg and for kicking with the right leg may both be true when considered as individual actions, yet they cannot be performed as concurrent actions. Specifying preconditions for concurrent actions is tricky because the preconditions that hold for executing individual actions can interact in complicated ways when applied for executing concurrent actions. Thus, sometimes even when the preconditions for two actions may be jointly consistent, intuitively they should not be concurrently possible. This problem is called the precondition interaction problem [Rei96].

The effect of pushing an object is opposite of the effect of pulling an object. Therefore, even though each action when performed in sequence can produce expected effects, when they are performed concurrently, their effects interfere. The interactions between effects are discussed in [BB01].