pragma solidity ^0.4.18;

contract ERC721Receiver {
  /// @dev Magic value to be returned upon successful reception of an NFT
  ///  Equals to bytes4(keccak256("onERC721Received(address,uint256,bytes)"))
  bytes4 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);
}