code: private long getChunkId(final Frame fr) {
        Key[] keys = new Key[fr.anyVec().nChunks()];
        for(int i = 0; i < fr.anyVec().nChunks(); ++i) {
          keys[i] = fr.anyVec().chunkKey(i);
        }
        for(int i = 0; i < keys.length; ++i) {
          if (keys[i].home()) return i;
        }
        return -99999; //throw new Error("No key on this node");
      }
docs: Return chunk index of the first chunk on this node. Used to identify the trees built here.