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