ERC721ownerOf.sol

pragma solidity =0.4.22;
/* Bad */
contract BadERC721Token{
   function ownerOf(uint256 _tokenId) external view returns (bool);
    //...
}

/* Better */
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/SafeERC721.sol";
import "https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/IERC721.sol";
contract GoodERC721Token {
  using SafeERC721 for IERC721;
  IERC721 public _token;
	function ownerOf(uint256 _tokenId) view returns (bool) {
		return _token.ownerOf(_tokenId);
	}
}