// ManyFields.nested_map_bool[true].string_field
  FieldIndex nested_map_bool_field_tag;
  nested_map_bool_field_tag.set_field(7);
  FieldIndex nested_map_bool_mapkey_field_tag;
  nested_map_bool_mapkey_field_tag.mutable_map_key()->set_boolean(true);
  FieldIndex string_field_field_tag;
  string_field_field_tag.set_field(3);
  RepeatedPtrField<FieldIndex> target_two_field_tags;
  target_two_field_tags.Add(FieldIndex(nested_map_bool_field_tag));