code: public static Backbone computeNegative(final Collection<Formula> formulas) {
          return compute(formulas, allVariablesInFormulas(formulas), BackboneType.ONLY_NEGATIVE);
      }
docs: Computes the negative backbone variables for a given collection of formulas.
  @param formulas the given collection of formulas
  @return the negative backbone or {@code null} if the formula is UNSAT