@Article{ORo89,
    Author       = "O'Rourke, P.",
    Year         = "1989",
    Title        = "{LT Revisited: Explanation-based Learning and the Logic of
                    Principia Mathematica}",
    Journal      = "Machine Learning",
    Volume       = "4",
    Pages        = "117-159",
    Comment      = "TPTPRef, general reasoning"
}