  We use testing to check if a combinational circuit $N$ always evaluates to 0
(written as $N \equiv 0$). We call a set of tests proving $N \equiv 0$ a
complete test set (CTS). The conventional point of view is that to prove $N
\equiv 0$ one has to generate a trivial CTS. It consists of all $2^{|X|}$ input
assignments where $X$ is the set of input variables of $N$. We use the notion
of a Stable Set of Assignments (SSA) to show that one can build a non-trivial
CTS consisting of less than $2^{|X|}$ tests. Given an unsatisfiable CNF formula
$H(W)$, an SSA of $H$ is a set of assignments to $W$ that proves
unsatisfiability of $H$. A trivial SSA is the set of all $2^{|W|}$ assignments
to $W$. Importantly, real-life formulas can have non-trivial SSAs that are much
smaller than $2^{|W|}$. In general, construction of even non-trivial CTSs is
inefficient. We describe a much more efficient approach where tests are
extracted from an SSA built for a `projection' of $N$ on a subset of variables
of $N$. These tests can be viewed as an approximation of a CTS for $N$. We give
experimental results and describe potential applications of this approach.
