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