public Builder mergePosition(org.box2d.proto.Box2D.PbVec2 value) {
        if (positionBuilder_ == null) {
          if (((bitField0_ & 0x00000004) == 0x00000004) &&
              position_ != org.box2d.proto.Box2D.PbVec2.getDefaultInstance()) {
            position_ =
              org.box2d.proto.Box2D.PbVec2.newBuilder(position_).mergeFrom(value).buildPartial();
          } else {
            position_ = value;
          }
          onChanged();
        } else {
          positionBuilder_.mergeFrom(value);
        }
        bitField0_ |= 0x00000004;
        return this;
      }