Sanjiang Li Mingsheng Ying Soft constraint abstraction based on semiring homomorphism. 192-201 2008 403 Theor. Comput. Sci. 2-3 https://doi.org/10.1016/j.tcs.2008.03.029 https://www.wikidata.org/entity/Q62042755 db/journals/tcs/tcs403.html#LiY08