code: private void init(final List<Integer> variables, final BackboneType type) {
          this.type = type;
          this.candidates = new Stack<>();
          this.assumptions = new LNGIntVector(variables.size());
          this.backboneMap = new HashMap<>();
          for (final Integer var : variables) {
              this.backboneMap.put(var, Tristate.UNDEF);
          }
      }
docs: Initializes the internal solver state.
  @param type      backbone type
  @param variables to test