Leitsch 1997, p. 11 "Before applying the inference method itself, we transform the formulas to quantifier-free conjunctive normal form." Leitsch, Alexander (1997). The Resolution Calculus. Texts in Theoretical Computer Science. An EATCS Series. Springer. ISBN978-3-642-60605-2.
Murray, Neil V. (February 1979). A Proof Procedure for Quantifier-Free Non-Clausal First Order Logic (Technical report). Electrical Engineering and Computer Science, Syracuse University. 39. (Cited from Manna, Waldinger, 1980 as: "A Proof Procedure for Non-Clausal First-Order Logic", 1978)