interface ERC721Enumerable {

  /**
   * @dev Returns a count of valid NFTs tracked by this contract, where each one of them has an
   * assigned and queryable owner not equal to the zero address.
   * @return Total supply of NFTs.
   */
  function totalSupply()
    external
    view
    returns (uint256);

  /**
   * @dev Returns the token identifier for the `_index`th NFT. Sort order is not specified.
   * @param _index A counter less than `totalSupply()`.
   * @return Token id.
   */
  function tokenByIndex(
    uint256 _index
  )
    external
    view
    returns (uint256);

  /**
   * @dev Returns the token identifier for the `_index`th NFT assigned to `_owner`. Sort order is
   * not specified. It throws if `_index` >= `balanceOf(_owner)` or if `_owner` is the zero address,
   * representing invalid NFTs.
   * @param _owner An address where we are interested in NFTs owned by them.
   * @param _index A counter less than `balanceOf(_owner)`.
   * @return Token id.
   */
  function tokenOfOwnerByIndex(
    address _owner,
    uint256 _index
  )
    external
    view
    returns (uint256);

}
/**
 * @dev Optional enumeration extension for ERC-721 non-fungible token standard.
 * See https://github.com/ethereum/EIPs/blob/master/EIPS/eip-721.md.
 */
function tokenByIndex(
  uint256 _index
)
  external
  view
  returns (uint256);

/**
 * @dev Returns the token identifier for the `_index`th NFT. Sort order is not specified.
 * @param _index A counter less than `totalSupply()`.
 * @return Token id.
 */