public static Backbone computeNegative(final Formula formula, final Collection<Variable> variables) {
        return compute(formula, variables, BackboneType.ONLY_NEGATIVE);
    }
public static Backbone computeNegative ( final Formula formula , final Collection < Variable > variables ) { return compute ( formula , variables , BackboneType . ONLY_NEGATIVE ) ; }
Computes the negative backbone variables for a given formula w.r.t. a collection of variables.
@param formula   the given formula
@param variables the given collection of relevant variables for the backbone computation
@return the negative backbone or {@code null} if the formula is UNSAT