contract ERC721Receiver {
  /**
   * @dev Magic value to be returned upon successful reception of an NFT
   *  Equals to `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`,
   *  which can be also obtained as `ERC721Receiver(0).onERC721Received.selector`
   */
  bytes4 constant ERC721_RECEIVED = 0xf0b9e5ba; 

  /**
   * @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. This function MUST use 50,000 gas or less. Return of other
   *  than the magic value MUST result in the transaction being reverted.
   *  Note: the contract address is always the message sender.
   * @param _from The sending address 
   * @param _tokenId The NFT identifier which is being transfered
   * @param _data Additional data with no specified format
   * @return `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`
   */
  function onERC721Received(address _from, uint256 _tokenId, bytes _data) public returns(bytes4);
}
function onERC721Received(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. This function MUST use 50,000 gas or less. Return of other
 *  than the magic value MUST result in the transaction being reverted.
 *  Note: the contract address is always the message sender.
 * @param _from The sending address 
 * @param _tokenId The NFT identifier which is being transfered
 * @param _data Additional data with no specified format
 * @return `bytes4(keccak256("onERC721Received(address,uint256,bytes)"))`
 */