public Builder mergeTarget(org.box2d.proto.Box2D.PbVec2 value) {
        if (targetBuilder_ == null) {
          if (((bitField0_ & 0x04000000) == 0x04000000) &&
              target_ != org.box2d.proto.Box2D.PbVec2.getDefaultInstance()) {
            target_ =
              org.box2d.proto.Box2D.PbVec2.newBuilder(target_).mergeFrom(value).buildPartial();
          } else {
            target_ = value;
          }
          onChanged();
        } else {
          targetBuilder_.mergeFrom(value);
        }
        bitField0_ |= 0x04000000;
        return this;
      }