Armin Hemmerling On P Versus NP for Parameter-Free Programs Over Algebraic Structures. 67-92 2001 47 Math. Log. Q. 1 http://dx.doi.org/10.1002/1521-3870(200101)47:1<67::AID-MALQ67>3.0.CO;2-V db/journals/mlq/mlq47.html#Hemmerling01