docs: public final Set<Variable> vars() {
          if (vars == null) {
              ImmutableSet.Builder<Variable> builder = ImmutableSet.<Variable>builder().add(start());
              Variable end = end();
              if (end != null) builder.add(end);
              builder.addAll(otherVars());
              vars = builder.build();
          }
  
          return vars;
      }