  Answer substitutions play a central role in logic programming. To support
{\it selective} answer substitutions, we refine $\exists x$ in goals into two
different versions: the noisy version $\exists^o x$ and the silent version
$\exists x$. The main difference is that only the instantiation in $\exists^o
x$ will be recorded in the answer substitutions. Similarly for $\forall x$. In
addition, we discuss the notion of don't-care constants and don't-know
constants.
