Since the ultimate purpose of a dialogue may be to accomplish something in the world, not only in the minds of the participants, reasoning, goal-directed planning, and action need to occur at the domain level as well. The goals of speech acts are then not ends in themselves, but means to other ends in the domain, perhaps to be accomplished by physical action (such as equipment repair). As a result, task-oriented dialogues are apt to be structured in a way that follows or “echoes” the structure of the domain entities and the way they can be purposefully acted upon. Such considerations led to Grosz and Sidner's theory of dialogue structure in task-oriented dialogues (Grosz and Sidner 1986). Their theory centers around the idea of shifts of attention mediated by pushing and popping of “focus spaces” on a stack. Focus spaces hold in them structured representations of the domain actions under consideration. For example, setting a collaborative goal of attaching a part to some device would push a corresponding focus space onto the stack. As dictated by knowledge about the physical task, the participants might next verbally commit to the steps of using a screwdriver and some screws to achieve the goal, and this part of the dialogue would be mediated by pushing corresponding subspaces onto the focus stack. When a subtask is achieved, the corresponding focus space is popped from the stack.