Egidio Astesiano Maura Cerioli Non-Strict Don't Care Algebras and Specifications. 121-142 1991 conf/tapsoft/1991-1 TAPSOFT, Vol.1 db/conf/tapsoft/tapsoft91-1.html#AstesianoC91 http://dx.doi.org/10.1007/3-540-53982-4_8