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