ERC721 remix error

gistfile1.txt

pragma solidity ^0.4.20;

import 'https://github.com/OpenZeppelin/zeppelin-solidity/contracts/token/ERC721/ERC721Token.sol';
import 'https://github.com/OpenZeppelin/zeppelin-solidity/contracts/ownership/Ownable.sol';


contract SubscriptionToken is ERC721Token, Ownable {
  string public constant name_ = "SubscriptionToken";
  string public constant symbol_ = "SUB";
  
  function name() public view returns (string _name) {
      return 'test';
  }
  function symbol() public view returns (string _symbol) {
      return 'test';
  }
  
  function tokenURI(uint256 _tokenId) public view returns (string) {
      return 'test';
  }
  
  function totalSupply() public view returns (uint256) {
      return 234;
  }
  function tokenOfOwnerByIndex(address _owner, uint256 _index) public view returns (uint256 _tokenId) {
      return 234;
  }
  function tokenByIndex(uint256 _index) public view returns (uint256) {
      return 234;
  }
}