[1][2] The Prolog system XSB implements the well-founded semantics since 1997.
[3][4] The well-founded semantics assigns a unique model to every general logic program.
However, instead of only assigning propositions true or false, it adds a third value unknown for representing ignorance.
Stratified logic programs have a 2-valued well-founded model, in which every proposition is either true or false.
[5] In 1989, Van Gelder suggested an algorithm to compute the well-founded semantics of a propositional logic program whose time complexity is quadratic in the size of the program.