abstract contract VersionedInitializable {
  /**
   * @dev Indicates that the contract has been initialized.
   */
  uint256 internal lastInitializedRevision = 0;

  /**
   * @dev Modifier to use in the initializer function of a contract.
   */
  modifier initializer() {
    uint256 revision = getRevision();
    require(revision > lastInitializedRevision, 'Contract instance has already been initialized');

    lastInitializedRevision = revision;

    _;
  }

  /// @dev returns the revision number of the contract.
  /// Needs to be defined in the inherited class as a constant.
  function getRevision() internal pure virtual returns (uint256);

  // Reserved storage space to allow for layout changes in the future.
  uint256[50] private ______gap;
}
/**
 * @title VersionedInitializable
 *
 * @dev Helper contract to support initializer functions. To use it, replace
 * the constructor with a function that has the `initializer` modifier.
 * WARNING: Unlike constructors, initializer functions must be manually
 * invoked. This applies both to deploying an Initializable contract, as well
 * as extending an Initializable contract via inheritance.
 * WARNING: When used with inheritance, manual care must be taken to not invoke
 * a parent initializer twice, or ensure that all initializers are idempotent,
 * because this is not dealt with automatically as with constructors.
 *
 * @author Aave, inspired by the OpenZeppelin Initializable contract
 */
function getRevision() internal pure virtual returns (uint256);

/// @dev returns the revision number of the contract.
/// Needs to be defined in the inherited class as a constant.