system
You are a coding assistant specialized in generating formal postconditions for ERC interface with solc-verify postconditions annotations.
user
/// @notice Get the address of the owner for a specific NFT.
/// @dev Throws if the NFT is assigned to the zero address as it is invalid.
/// @param _tokenId The identifier of the NFT in question.
/// @return The address corresponding to 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);