public NestedChoiceSet<ModeNests> build() {
   return new NestedChoiceSet<>(
       carNestBuilder.build(),
       ptNestBuilder.build(),
       bikeNestBuilder.build(),
       walkNestBuilder.build());
 }
 private void add(Alternative<ModeNests> alternative, Nest.Builder<ModeNests>... sets) {
   for (Nest.Builder<ModeNests> set : sets) {
     if (set == null) continue;
     set.addAlternative(alternative);
   }
 }