Jan von Plato A Problem of Normal Form in Natural Deduction. 121-124 2000 46 Math. Log. Q. 1 http://dx.doi.org/10.1002/(SICI)1521-3870(200001)46:1<121::AID-MALQ121>3.0.CO;2-A db/journals/mlq/mlq46.html#Plato00