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.
 */
function supportsInterface(bytes4 interfaceId) external view returns (bool) {
    return _supportedInterfaces[interfaceId];
}

/**
 * @dev See {IERC165-supportsInterface}.
 *
 * Time complexity O(1), guaranteed to always use less than 30 000 gas.
 */