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