Well-founded semantics

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The well-founded semantics is a three-valued version of the stable semantics. That is, instead of only assigning propositions true or false, it also allows for a value representing ignorance. For example, if we know that

Specimen A is a moth if specimen A does not fly during daylight.

but we do not know whether or not specimen A flies during the day, the well-founded semantics would assign the proposition ``specimen A is a moth`` the value bottom which is neither true nor false.

The well-founded semantics is also a way of making safe inferences in the presence of contradictory data such as noisy data, or data acquired from different experts who may posit differing opinions. Many two-valued semantics simply won't consider such a problem state workable. The well-founded semantics, however, has a built-in mechanism to circumvent the presence of the contradictions and proceeds in the best way that it can. The fastest known algorithm to compute the WF-Semantics in general, is of quadratic complexity.