contract ERC721Tradable is ERC721, Ownable {
  using Strings for string;

  address proxyRegistryAddress;
  uint256 private _currentTokenId = 0;


  constructor(string memory _name, string memory _symbol, address _proxyRegistryAddress) ERC721(_name, _symbol) public {
    proxyRegistryAddress = _proxyRegistryAddress;
  }



  /**
    * @dev calculates the next token ID based on value of _currentTokenId
    * @return uint256 for the next token ID
    */
  function _getNextTokenId() internal view returns (uint256) {
    return _currentTokenId.add(1);
  }

  /**
    * @dev increments the value of _currentTokenId
    */
  function _incrementTokenId() internal  {
    _currentTokenId++;
  }


  /**
   * Override isApprovedForAll to whitelist user's OpenSea proxy accounts to enable gas-less listings.
   */
  function isApprovedForAll(
    address owner,
    address operator
  )
    public
    view
    override
    returns (bool)
  {
    // Whitelist OpenSea proxy contract for easy trading.
    ProxyRegistry proxyRegistry = ProxyRegistry(proxyRegistryAddress);
    if (address(proxyRegistry.proxies(owner)) == operator) {
        return true;
    }

    return super.isApprovedForAll(owner, operator);
  }
}
/**
 * @title ERC721Tradable
 * ERC721Tradable - ERC721 contract that whitelists a trading address, and has minting functionality.
 */
function isApprovedForAll(
  address owner,
  address operator
)
  public
  view
  override
  returns (bool)
{
  // Whitelist OpenSea proxy contract for easy trading.
  ProxyRegistry proxyRegistry = ProxyRegistry(proxyRegistryAddress);
  if (address(proxyRegistry.proxies(owner)) == operator) {
      return true;
  }

  return super.isApprovedForAll(owner, operator);
}

/**
 * Override isApprovedForAll to whitelist user's OpenSea proxy accounts to enable gas-less listings.
 */