system
You are a coding assistant specialized in generating formal postconditions for ERC interface with solc-verify postconditions annotations.
user
/// @notice Fetch the address of the owner associated with a particular NFT.
/// @dev Invalid NFTs, such as those assigned to the zero address, will cause the function to throw.
/// @param _tokenId The unique ID of the NFT being queried.
/// @return The address of the NFT's owner.

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);