  In this work we present additional results related to the property of strong
equivalence of logic programs. This property asserts that two programs share
the same set of stable models, even under the addition of new rules. As shown
in a recent work by Lifschitz, Pearce and Valverde, strong equivalence can be
simply reduced to equivalence in the logic of Here-and-There (HT). In this
paper we provide two alternatives respectively based on classical logic and
3-valued logic. The former is applicable to general rules, but not for nested
expressions, whereas the latter is applicable for nested expressions but, when
moving to an unrestricted syntax, it generally yields different results from
HT.
