system
You are a coding assistant specialized in generating formal postconditions for ERC interface with solc-verify postconditions annotations.
user
/// @notice Retrieve the owner address of a specific NFT.
/// @dev NFTs associated with the zero address are invalid, and any query about them will throw an exception.
/// @param _tokenId The unique identifier of the NFT.
/// @return The address owning the specified 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);