contract ERC165 is IERC165 {
  /*
   * bytes4(keccak256('supportsInterface(bytes4)')) == 0x01ffc9a7
   */
  bytes4 private constant _INTERFACE_ID_ERC165 = 0x01ffc9a7;

  /**
   * @dev Mapping of interface ids to whether or not it's supported.
   */
  mapping(bytes4 => bool) private _supportedInterfaces;

  constructor () internal {
    // Derived contracts need only register support for their own interfaces,
    // we register support for ERC165 itself here
    registerInterface(_INTERFACE_ID_ERC165);
  }

  /**
   * @dev See {IERC165-supportsInterface}.
   *
   * Time complexity O(1), guaranteed to always use less than 30 000 gas.
   */
  function supportsInterface(bytes4 interfaceId) external view returns (bool) {
    return _supportedInterfaces[interfaceId];
  }

  /**
   * @dev Registers the contract as an implementer of the interface defined by
   * `interfaceId`. Support of the actual ERC165 interface is automatic and
   * registering its interface id is not required.
   *
   * See {IERC165-supportsInterface}.
   *
   * Requirements:
   *
   * - `interfaceId` cannot be the ERC165 invalid interface (`0xffffffff`).
   */
  function registerInterface(bytes4 interfaceId) internal {
    require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
    _supportedInterfaces[interfaceId] = true;
  }
}
/**
 * @dev Implementation of the {IERC165} interface.
 *
 * Contracts may inherit from this and call {_registerInterface} to declare
 * their support of an interface.
 * Source: https://github.com/OpenZeppelin/openzeppelin-contracts/blob/master/contracts/introspection/ERC165.sol
 */
function registerInterface(bytes4 interfaceId) internal {
    require(interfaceId != 0xffffffff, "ERC165: invalid interface id");
    _supportedInterfaces[interfaceId] = true;
  }
/**
 * @dev Registers the contract as an implementer of the interface defined by
 * `interfaceId`. Support of the actual ERC165 interface is automatic and
 * registering its interface id is not required.
 *
 * See {IERC165-supportsInterface}.
 *
 * Requirements:
 *
 * - `interfaceId` cannot be the ERC165 invalid interface (`0xffffffff`).
 */