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