contract ERC721Receiver {
  /**
   * @dev Magic value to be returned upon successful reception of an NFT
   *  Equals to `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`,
   *  which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
   */
  bytes4 internal constant ERC721_RECEIVED = 0x150b7a02;

  /**
   * @notice Handle the receipt of an NFT
   * @dev The ERC721 smart contract calls this function on the recipient
   * after a `safetransfer`. This function MAY throw to revert and reject the
   * transfer. Return of other than the magic value MUST result in the
   * transaction being reverted.
   * Note: the contract address is always the message sender.
   * @param _operator The address which called `safeTransferFrom` function
   * @param _from The address which previously owned the token
   * @param _tokenId The NFT identifier which is being transferred
   * @param _data Additional data with no specified format
   * @return `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
   */
  function onERC721Received(
    address _operator,
    address _from,
    uint256 _tokenId,
    bytes _data
  )
    public
    returns(bytes4);
}
/**
 * @title ERC721 token receiver interface
 * @dev Interface for any contract that wants to support safeTransfers
 * from ERC721 asset contracts.
 */
function onERC721Received(
    address _operator,
    address _from,
    uint256 _tokenId,
    bytes _data
  )
    public
    returns(bytes4);
/**
 * @notice Handle the receipt of an NFT
 * @dev The ERC721 smart contract calls this function on the recipient
 * after a `safetransfer`. This function MAY throw to revert and reject the
 * transfer. Return of other than the magic value MUST result in the
 * transaction being reverted.
 * Note: the contract address is always the message sender.
 * @param _operator The address which called `safeTransferFrom` function
 * @param _from The address which previously owned the token
 * @param _tokenId The NFT identifier which is being transferred
 * @param _data Additional data with no specified format
 * @return `bytes4(keccak256("onERC721Received(address,address,uint256,bytes)"))`
 */