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          "Language restrictions",
          "How the operators are given",
          "Allow delete lists?",
          "Allow negated preconditions?",
          "Telling if a plan exists",
          "Telling if there’s a plan of length ≤ k"
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      "text": "ᵅNo operator has more than one precondition.",
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      "text": "ᵞWith PSPACE- or NP-completeness for some sets of operators.",
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      "text": "ᵟThese results are due to Bylander (1991). All other results are new.",
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      "text": "This is because what makes the problem hard is the task of choosing operators that achieve several subgoals in order to minimize the overall length of the plan, and this task remains equally hard regardless of whether negated preconditions are allowed.",
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      "text": "Let ℒ be any datalog (i.e., function-free first-order) language. Then a state is any nonempty set of ground atoms in ℒ. Intuitively, a state tells us which ground atoms are currently true. Thus, if a ground atom A is in state S, then A is true in state S; if B ∉ S, then B is false in state S. Thus, a state is simply an Herbrand interpretation for the language ℒ, and hence each formula of first-order logic is either satisfied or not satisfied in S according to the usual first-order logic definition of satisfaction.",
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      "text": "Language. The language ℒ contains a supply of variable symbols X₁, X₂, …, and three constant symbols a, b, c to represent the three blocks. ℒ contains a binary predicate symbol “on”, unary predicate symbols “ontable”, “clear”, and “holding”, and a 0-ary",
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