* returns a singleton set containing the formula itself.
*/
public static Set<Formula> conjuncts(Formula formula) {
if (formula instanceof BinaryFormula) {
final BinaryFormula bin = (BinaryFormula) formula;
if (bin.op()==FormulaOperator.AND) {
final Formula left = bin.left(), right = bin.right();
if (left==right) return Collections.singleton(left);
else return new AbstractSet<Formula>() {
@Override
public boolean contains(Object o) { return left==o || right==o; }