code: public static Backbone compute(final Collection<Formula> formulas, final BackboneType type) {
          return compute(formulas, allVariablesInFormulas(formulas), type);
      }
docs: Computes the backbone for a given collection of formulas w.r.t. a given backbone type.
  @param formulas the given collection of formulas
  @param type     the type of backbone variables that should be computed
  @return the backbone or {@code null} if the formula is UNSAT