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