Users typically navigate the state space of a task through the explicit manipulation of its constituent elements. This paper proposes the Principle of State Expansion (PoSE), a novel alternative to such explicit manipulation. Under the PoSE, users manipulate a collection of states and navigate through the state space by selecting states to expand. An abstract task is presented that contrasts the approaches and demonstrates the generality of the PoSE. This is followed by discussion of WordBird and X-Panda, two concrete implementations of software that follow the PoSE. In conclusion, the advantages over explicit manipulation are discussed especially with reference to the potential for sophisticated learner modelling.
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