code: private List<Integer> getRelevantVarIndices(final Collection<Variable> variables) {
          final List<Integer> relevantVarIndices = new ArrayList<>(variables.size());
          for (final Variable var : variables) {
              final Integer idx = this.name2idx.get(var.name());
              // Note: Unknown variables are variables added to the solver yet. Thus, these are optional variables and can
              // be left out for the backbone computation.
              if (idx != null) {
                  relevantVarIndices.add(idx);
              }
          }
          return relevantVarIndices;
      }
docs: Returns a list of relevant variable indices. A relevant variable is known by the solver.
  @param variables variables to convert and filter
  @return list of relevant variable indices