update erc721

contracts/intf/IERC721Enumerable.sol

@@ -0,0 +1,32 @@
+// This is a file copied from https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/token/ERC721/extensions/IERC721Enumerable.sol
+// SPDX-License-Identifier: MIT
+
+pragma solidity 0.6.9;
+
+import "./IERC165.sol";
+import "./IERC721.sol";
+
+
+/**
+ * @title ERC-721 Non-Fungible Token Standard, optional enumeration extension
+ * @dev See https://eips.ethereum.org/EIPS/eip-721
+ */
+interface IERC721Enumerable is IERC721 {
+
+    /**
+     * @dev Returns the total amount of tokens stored by the contract.
+     */
+    function totalSupply() external view returns (uint256);
+
+    /**
+     * @dev Returns a token ID owned by `owner` at a given `index` of its token list.
+     * Use along with {balanceOf} to enumerate all of ``owner``'s tokens.
+     */
+    function tokenOfOwnerByIndex(address owner, uint256 index) external view returns (uint256 tokenId);
+
+    /**
+     * @dev Returns a token ID at a given `index` of all the tokens stored by the contract.
+     * Use along with {totalSupply} to enumerate all tokens.
+     */
+    function tokenByIndex(uint256 index) external view returns (uint256);
+}