blobdata get_block_hashing_blob(const block& b)
  {
    blobdata blob = t_serializable_object_to_blob(static_cast<block_header>(b));
    crypto::hash tree_root_hash = get_tx_tree_hash(b);
    blob.append(reinterpret_cast<const char*>(&tree_root_hash), sizeof(tree_root_hash));
    blob.append(tools::get_varint_data(b.tx_hashes.size()+1));
    return blob;
  }