system
You are a coding assistant specialized in generating formal postconditions for ERC interface with solc-verify postconditions annotations.
user
/// @notice Find the owner of an NFT
/// @dev NFTs assigned to zero address are considered invalid, and queries
///  about them do throw.
/// @param _tokenId The identifier for an NFT
/// @return The address of the owner of the NFT

function ownerOf(uint256 _tokenId) external view returns (address);
assistant
/// @notice postcondition _tokenOwner[_tokenId] == _owner
/// @notice postcondition  _owner !=  address(0)
function ownerOf(uint256 _tokenId) public view returns (address owner);