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