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Discussiones Mathematicae - General Algebra and Applications > 2003 > 23 > 1 > 31-43
Demonstratio Mathematica > 1999 > Vol. 32, nr 1 > 1-11
Discussiones Mathematicae - General Algebra and Applications > 2003 > 23 > 1 > 31-43
Demonstratio Mathematica > 1999 > Vol. 32, nr 1 > 1-11